Why don't microwaves leak out from through the glass? How a microwave oven works Controller Sensors Cooling fans Microwave generator Magnetron construction and operation Magnetron construction - basic textbook description Magnetron construction - modern microwave oven Magnetron construction - common features Cross section diagram of typical magnetron Microwave generator circuit diagram Interlock switches Troubleshooting Guide Instant troubleshooting chart - most common problems and possible causes What can go wrong General system problems Uninvited guests Totally dead oven Totally dead oven after repair Dead controller No response to any buttons on touchpad Oven runs when door is still open Oven starts on its own as soon as door is closed Oven works but totally dead display Whacked out controller or incorrect operation Erratic behavior Problems with internal microwave leakage Some of the keys on the touchpad do not function or perform the wrong action Microwave oven does not respond to START button Microwave generator problems No heat but otherwise normal operation Timer and light work but no heat, cooling fan, or turntable rotation Fuse blows when closing or opening door Loud hum and/or burning smell when attempting to cook Arcing in or above oven chamber More on the waveguide cover and cleaning Fuse blows when initiating cook cycle Fuse blows when microwave shuts off (during or at end of cook cycle) Oven heats on high setting regardless of power setting Oven heats but power seems low or erratic Oven heats but shuts off randomly Oven makes (possibly erratic) buzzing noise when heating Oven light does not work Fans or turntables that do not work What to do if the door handle breaks off Crack or other damage to door window Repairing damage to the oven interior Microwave/convection oven problems Sensor problems Testing and Replacement of Components Testing the oven - the water heating test Testing the main fuse

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Notes on the Troubleshooting and Repair of Microwave Ovens

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Testing and replacing of interlock switches Making measurements inside microwave ovens Testing the high voltage components Testing the high voltage diode Replacing the HV diode HV diode ratings Testing the high voltage capacitor Replacing the high voltage capacitor What if the HV diode or capacitor are leaky? Testing the magnetron Comprehensive list of magnetron failure modes Where to obtain replacement magnetrons Comments on replacement magnetron quality Replacing the magnetron Testing the high voltage transformer Testing the HV transformer using an AC current meter Replacing the high voltage transformer Testing and repairing the wiring and connections Testing thermal protectors and thermal fuses Testing and replacing the triac Testing and replacing the power relay Items of Interest Microwave leakage meters Comments on microwave leakage meters Simple microwave leak detectors How safe is a repaired microwave oven? Efficiency of microwave ovens Microwave oven design and cost reduction Problems with running a microwave oven with metal inside or totally empty More on metal in the microwave Burnt smell from oven - after incident Microwave ovens and grounded dedicated circuits Microwave ovens and GFCIs Can a microwave oven be built into (or hung under) a cabinet? Taking a microwave oven oversees (or vice versa) Microwave oven test-mode High frequency inverter type HV power supplies Dangerous (or useful) parts in a dead microwave oven? The magnets in dead magnetrons Using the control panel from defunct microwave oven as an electronic timer Precise control of microwave oven power Has technology gone too far? Microwave ovens for non-standard applications Short course on Amana Computer system near microwave oven? Why Microwave-Safe Containers Get Destroyed Service Information Advanced troubleshooting Suggested Reference Cost of repair parts Interchangeability of components Can I substitute a slightly different HV capacitor for a blown one? Obtaining replacement parts for microwave ovens

DISCLAIMER
Careless troubleshooting of a microwave oven can result in death or worse. Experienced technicians have met their maker as a result of a momentary lapse of judgement while testing an oven with the cover removed. Microwave ovens are without a doubt, the most deadly type of consumer electronic equipment in wide spread use. The power supplies for even the smallest microwave ovens operate at extremely lethal voltage and current levels. Do not attempt to troubleshoot, repair, or modify such equipment without understanding and following ALL of the relevant safety guidelines for high voltage and/or line connected electrical and electronic systems. We will not be responsible for damage to equipment, your ego, county wide power outages, spontaneously generated mini (or larger) black holes, planetary disruptions, or personal injury or worse that may result from the use of this material.

Back to Microwave Oven Repair FAQ Table of Contents.

Introduction
Radar Range anyone?
Remember when you actually had to use the real oven to defrost a TV dinner? Think back - way back - before VCRs, before PCs (and yes, before Apple computers as well), almost before dinosaurs, it would seem. There was a time when the term 'nuke' was not used for anything other than bombs and power reactors.

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you will have the satisfaction of knowing you did as much as you could before taking it in for professional repair. This document provides maintenance and repair information applicable to most of the microwave ovens in existence. Not entirely coincidentally. you will have the upper hand and will not easily be snowed by a dishonest or incompetent technician.and at performance levels indistinguishable from when it was first taken out of the box.org/sam/micfaq. some of its wording appears remarkably familiar! Microtech also offers instructional videos and books on microwave oven and VCR repair. you can greatly simplify your troubleshooting or at least confirm a diagnosis before ordering parts. Touchpads are now nearly universal because they are cheaper to manufacture than mechanical timers (and also more convenient). There are also an extensive list of microwave oven related links to other interesting sites (including this document!). specific manufacturers and models will not be covered as there are so many variations that such a treatment would require a huge and very detailed text.repair. Whether these issues have been resolved or just brushed aside is not totally clear. The comprehensive Safety Info is a must read as well. Should you still not be able to find a solution. Jim Bryant's Microwave Ovens page is another site worth visiting.be able to revive something that would otherwise have gone into the dumpster or continued in its present occupation as a door stop or foot rest. Nonetheless. With your new-found knowledge. You will be able to decide if it is worth the cost of a repair as well. While he deals mostly with models in the UK. In many cases. However. In any case. it is difficult to deny the convenience and cooking speed that is provided by this relatively simple appliance. In that case. Connoisseurs of fine dining will turn up their collective noses at the thought of using a microwave oven for much beyond boiling water .this has nothing to do with Microtech in particular . There is a good chance that your oven will operate for 10 years or more without requiring repairs of any kind .is that symptoms can sometimes be deceiving and a solution that works in one instance may not apply to your specific problem.the microwave oven has not changed substantially in the last 20 years.electronics.repairfaq. Unlike other consumer electronics where a new model is introduced every 20 minutes .htm
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. an understanding of the hows and whys of the equipment along with some good old fashioned testing is highly desirable to minimize the risk of replacing parts that turn out not to be bad. he will answer questions via email and includes links to many USA microwave oven manufacturers and parts suppliers. My only reservation with respect to tech tips databases in general . You will be able to make an informed decision as to whether a new oven is the better alternative.
http://www.Notes on the Troubleshooting and Repair of Microwave Ovens
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For a long time. there was controversy as to whether microwave ovens were safe . Cooking is cooking. It is quite possible your problem is already covered at the Microtech site. you will have learned a great deal and be able to ask appropriate questions and supply relevant information if you decide to post to sci. Microwave ovens are extremely reliable devices. However.in terms of microwave emissions and molecular damage to the food. you will be able to do what is required for a fraction of the cost that would be charged by a repair center . Rather. the microwave oven has taken its place in virtually every kitchen on the planet.or .
On-line microwave oven repair database
Microtech maintains a web site with a large amount of information on microwave oven repair including an on-line Tech Tips Database with hundreds of solutions to common problem for many models of microwave ovens. It will enable you to quickly determine the likely cause and estimate the cost of parts. I assume.if that. an old microwave oven will heat foods just as well as a brand new one. Therefore.some even have useful improvements . the most common problems will be addressed and enough basic principles of operation will be provided to enable you to narrow the problem down and likely determine a course of action for repair. With minor exceptions.

Blown fuse due to power surge or old age: Replace fuse. replace MOV for future surge protection. Many problems can be solved quickly and inexpensively.
The simplest problems
Bad interlocks switches or door misalignment causing fuses to blow or no operation when the start button is pressed. Replace carbonized or damaged waveguide cover. An MOV.
Installation and Preventive Maintenance
Microwave oven installation and use
http://www. It will take you through a customized step-by-step procedure based on your symptoms (and specific microwave oven model in some cases) and the results of its suggested tests. You will learn a lot in the process. However. Arcing in oven chamber: clean oven chamber and waveguide thoroughly. Bugs in the works .. Touch up the interior paint. this may be an effective way of obtaining a solution quickly as long as you follow the extremely important safety information provided by MIDES (or this document). replacement should be considered seriously before sinking a large investment into an older oven.org/sam/micfaq. Thus. Fixing an old microwave for the dorm room may just make sense after all.Notes on the Troubleshooting and Repair of Microwave Ovens
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Expert system for microwave oven fault diagnosis
The MIDES (Microwave Oven Diagnosis Expert System) site represents an interesting and possibly useful approach for isolating the cause of many common failures. probably on the controller. Locate and replace defective switches and/or realign door. the equation changes dramatically as your parts costs will be 1/2 to 1/4 of what a professional will charge and of course your time is free.repairfaq.let touchpad dry out for a week. Erratic touchpad operation due to spill . The educational aspects may also be appealing. For the novice. Even full size microwave ovens with full featured touchpanel can be had for under $200. On rare occasions. if you can do the repair yourself... Smooth rough metal edges.
Back to Microwave Oven Repair FAQ Table of Contents. and followed their safety precautions and warnings before performing each test.
Repair or replace?
With small to medium size microwave ovens going for $60-100 it hardly makes sense to spend $60 to have one repaired.the controller circuit board is a nice warm safe cozy place to raise a family. the main fuse may even be intermittent causing very strange symptoms. You will not be forced to acknowledge that you have read. More detailed explanations are provided elsewhere in this document. may have shorted due to a power surge blowing the controller fuse. understood.. replace fuse and test.htm
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. Remove remains of MOV.

Select a stand-alone unit rather than a built-in if possible. It is very easy to cause a fire through the use of excessive times or power settings. however). regular cleaning at least will avoid potentially expensive repairs in the future: Clean the interior of the oven chamber after use with a damp cloth and some detergent if necessary.org/sam/micfaq. If there is any chance of food deposits having made their way above the waveguide cover in the roof of the chamber. Built up food deposits can eventually carbonize resulting in sparks. Make sure the outlet is in good condition in either case. cheaper and easier to service. spread the metal strips of each of the prongs apart if possible and/or replace the outlet. In any case. While not much is needed.as well as potentially more serious damage to the magnetron. It will be cheaper to buy. and electrical parts distributors. Temporary use of a 3 to 2 prong adapter is acceptable only if the outlet box is properly grounded to begin with (BX. If there is. arcs. Leave at least 2 inches on all sides and top if possible.
Microwave oven maintenance
Most people do not do anything to maintain a microwave oven. Microwave ovens are high power devices and a separate circuit will eliminate nuisance fuse blowing or circuit breaker tripping when multiple appliances are being used at the same time. remove the waveguide cover and
http://www. Allow adequate ventilation . heating.Notes on the Troubleshooting and Repair of Microwave Ovens
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To assure safety and convenient. Even something as simple as microwave popcorn can explode and/or catch fire if heated for too long . Select a convenient location .easy access and not too high or too low.. Do not let children use the microwave oven unless properly supervised.htm
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. What a concept! If nothing else. and possibly more reliable since ventilation and adjacent heat producing appliances will not be as much of a factor. follow these recommendations: Read your users manual from cover to cover especially if this is your first microwave.e.do not push it up against the wall or wedge it under a tight fitting wall cabinet (or inside one for that matter!). Inexpensice outlet testers are available at hardware stores. there may be requirements or suggestions that are specific to your model and will enable you to get the most performance from your new microwave. 5 minutes instead of my precisely determined 3:41 on high :-). Check that the plug (or adapter) fits tightly and that there is no appreciable heating of the outlet during use of the microwave oven. Put the microwave oven on its own dedicated 3 wire grounded circuit. This is particularly important if the door of the oven opens down instead of to the left side (only a few models are built this way. and damage to the mica waveguide cover and interior paint . home centers.repairfaq. A GFCI is not needed as long as the outlet is properly grounded and may result in nuisance tripping with some microwave ovens. to confirm that the outlet is properly wired and grounded. or conduit with ground) AND the adapter's ground wire or terminal is securely attached to the outlet box ground screw.g. Romex. you may discover that your oven has features you were not aware were even possible. A grounded outlet is essential for safety. It will also minimize the possibility of Radio Frequency Interference (RFI) between it and any electronic equipment which might be on the same circuit.

2. Attending to these minor problems now may prevent major repairs in the future. I know it is difficult to get at but I warned you about that!). However. squealing.org/sam/micfaq. WARNING: See the section: SAFETY before going inside. If it is too late and you have a recurring problem of cockroaches getting inside the electronics bay. immediately unplug the oven and let it dry out for a day or two. Keeping the ventilation free will minimize the chance of overheating. clean up whatever is attracting the unwanted tenants (and anything they may have left behind including their eggs!!). Clean them up and use a vacuum cleaner to suck up loose dust. There is little energy storage in the microwave generator compared to the amount being used. Listen for any unusual sounds coming from inside the oven. especially from a combination microwave/convection oven or from other heat producing appliances can damage the plug and/or cord. While these appliances are not exactly quiet. for example). DO NOT use a spray where any can find its way inside through the door latch or ventilation holes. Do not use strong solvents (though a bit of isopropyl alcohol is fine if needed to remove sticky residue from unwanted labels. scraping. Be especially careful around the area of the touchpad since liquid can seep underneath resulting in unresponsive or stuck buttons or erratic operation. or other noises . Yes. Inspect the cord and plug for physical damage and to make sure the plug is secure and tight in the outlet . There is no such thing as residual microwave radiation from a microwave oven . This is because: 1. or a dripping wet cloth. Periodically check for built up dust and dirt around the ventilation holes or grills. Once the beep has sounded (or the door has opened). Heat.or anywhere else inside.may indicate the need for some more extensive maintenance like belt replacement or motor lubrication. I know. Such an open mesh should not affect the cooling of the electronic components significantly. tell them to get lost and then put window screen over the vents (or wherever they are entering). for that matter. the outlet (and possibly the plug as well) should be replaced.particularly if the unit is installed inside a cabinet (yes. If there is evidence of overheating at the outlet itself. CAUTION: Do not spray anything into the holes where the door latch is inserted or anywhere around the touchpad as this can result in internal short circuits and costly damage .
How long does microwave energy hang around?
You have probably been warned by your mother: "Wait a few seconds (or minutes) after the beep for all the microwaves to disappear". the mesh will likely clog up more quickly than the original louvers so make sure it is cleaned regularly.especially if they were not there when the oven was new . it is safe.Notes on the Troubleshooting and Repair of Microwave Ovens
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thoroughly clean inside the waveguide as well. There is no scientific basis for such a recommendation.it is either being produced or is non-existent. Clean the exterior of the cabinet and touchpad in a similar manner. this isn't exactly microwave specific but cockroaches and other uninvited guests might just like to take up residence inside the electronics bay of the oven on the nice warm controller circuit board or its neighborhood and they aren't generally the tidiest folks in the world. If possible.htm
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.repairfaq. grinding. Keep your kitchen clean. If you do this by accident.
http://www.

for a 1500 W oven with a capacitor storing 15 W-s.the only component that can store energy .once the cabinet cover is removed. If you must probe live. In this case. See the troubleshooting sections later in this document. Power consumption is typically 800 to 1500 W depending on oven size. (Based on the numbers. the capacitor will be fully drained in much less than . Therefore.htm
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.electrical connections. Please see Typical Microwave Oven Electronics Bay for parts identification. Discharge the high voltage capacitor (with the oven unplugged) and then use clip leads to make any connections before you plug it in and apply power. there is a very real risk of potentially lethal electrical shock even after several minutes or more of being unplugged! See the section: SAFETY if you will be troubleshooting a microwave oven. Then after removing power and unplugging the oven discharge the HV capacitor once again. Always ensure that it is totally discharged before even thinking about touching or probing anything in the high voltage power circuits.
Back to Microwave Oven Repair FAQ Table of Contents. above. You may also be exposed to potentially harmful levels of microwave emissions if you run the oven with the cover off and there is damage or misalignment to the waveguide to the oven chamber.a high voltage capacitor .deadly combination. WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! WARNING! Microwave ovens are probably the most dangerous of consumer appliances to service.has a capacity of less than 15 W-s (Watt-seconds) even for the largest ovens.long before the beep has ended or the door has cleared the front panel.org/sam/micfaq.
Microwave Oven Troubleshooting
SAFETY
The following applies to microwave oven troubleshooting .
http://www. Very high voltages (up to 5000 V) at potentially very high currents (AMPs) are present when operating . There is also safety information on proper use of the oven in subsequent sections.1 second . There is a high voltage capacitor in the microwave generator. below.01 seconds!) WARNING: This only applies to a *working* microwave oven! If there is no heat. unplug the oven from the AC outlet before removing the cover and do not plug it in to operate it with the cover off if at all possible.that can retain a dangerous charge for a long time.Notes on the Troubleshooting and Repair of Microwave Ovens
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The typical high voltage capacitor . To prevent the possibility of extremely dangerous electric shock. remove the connections to the magnetron (see below) to prevent the inadvertent generation of microwaves except when this is absolutely needed during troubleshooting.repairfaq. Careless troubleshooting of a microwave oven can not only can fry you from high voltages at relatively high currents but can microwave irradiate you as well. replace the oven or have it professionally repaired.potentially lethal . the magnetron may not be drawing any current from the HV power supply and the HV capacitor can remain charged for a long time. If you have the slightest doubts about your knowledge and abilities to deal with these hazards. it is more like . When you remove the metal cover of the microwave oven you expose yourself to dangerous . These dangers do not go away even when unplugged as there is an energy storage device .

solder. or get caught in moving parts. discharge (across) large power supply filter capacitors with a 25 W or greater resistor of 5 to 50 ohms/V approximate value. put insulating material between the boards and anything they may short to.25 AMP at 50/60 Hz . The microwave oven circuitry is especially hazardous because the return for the high voltage is the chassis .plastic or wood.the continuous power rating of the HV transformer may exceed 1500 W with short term availability of much greater power. Always observe high voltage protocol. Therefore. Repair of TVs. The purpose of this set of guidelines is not to frighten you but rather to make you aware of the appropriate precautions. I highly recommend avoiding any probing of the HV circuits . do not assume that the chassis is a suitable ground for your test equipment! If circuit boards need to be removed from their mountings. particularly through your heart. while perhaps harmless in themselves.htm
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. Prop them up with insulation sticks . to be doubly sure that the capacitor if fully discharged. If you need to probe.Notes on the Troubleshooting and Repair of Microwave Ovens
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WARNING: Experienced technicians have been electrocuted deader than a brick from even careful probing of the HV circuits of a powered microwave oven. may cause collateral damage . Microwave ovens use the chassis as ground return for the high voltage. monitors. or otherwise touch circuits with power off. Any involuntary muscle contractions caused by a shock.org/sam/micfaq. Wear rubber bottom shoes or sneakers.in the event of an emergency another person's presence may be essential. the HV may exceed 5000 V peak with a continuous current rating of over . short across its terminals with the blade of a well insulated screwdriver. Touch each of the capacitor terminals to the non-grounded end of the resistor for several seconds.it is not isolated. Don't wear any jewelry or other articles that could accidentally contact circuitry and conduct current.repairfaq. Then.nearly everything can be determined by inspection and component tests with the oven unplugged. For the microwave oven in particular. Always keep one hand in your pocket when anywhere around a powered line-connected or high voltage system. I also recommend leaving a clip lead shorting across the capacitor
http://www. Safety guidelines These guidelines are to protect you from potentially deadly electrical shock hazards as well as the equipment from accidental damage. Set up your work area away from possible grounds that you may accidentally contact. Know your equipment: TVs and monitors may use parts of the metal chassis as ground return yet the chassis may be electrically live with respect to the earth ground of the AC line. Hold them in place with string or electrical tape. Note that the danger to you is not only in your body providing a conducting path. In addition. In addition. use a 25K to 100K 25 W resistor with a secure clip lead to the chassis. Just be sure that it is also safe! Don't work alone . microwave ovens. Mount the resistor on the end of a well insulated stick.there are many sharp edges inside this type of equipment as well as other electrically live parts you may contact accidentally. and other consumer and industrial equipment can be both rewarding and economical.

the semiconductors in the power supply section of a TV or monitor can be tested for short circuits with an ohmmeter. never assume anything without checking it out for yourself! Don't take shortcuts! As noted. connect the
http://www. Clip the reference end of the meter or scope to the appropriate ground return so that you need to only probe with one hand.will not be operating at full capacity.htm
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. However. or brush up against. At most. If you insist on making live measurements. put electrical tape over all but the last 1/16" of the test probes to avoid the possibility of an accidental short which could cause damage to various components.it's almost never needed in any case. Failures are usually easily found by performing test with the oven unplugged. it won't be terribly useful as noted above. A GFCI may. Perform as many tests as possible with power off and the equipment unplugged. Disconnect the HV transformer to eliminate the possibility of high voltage shock and to reduce the load. but your primary diagnostic tool . your equipment. lean on. Use clip leads or solder temporary wires to reach cramped locations or difficult to access locations. Connect/disconnect any test leads with the equipment unpowered and unplugged.org/sam/micfaq. A Variac(tm) is not an isolation transformer! (See the next section with regards to isolation transformers and microwave ovens. Not only will you be more careless. However. however. of course. the best policy is to NEVER EVER attempt to measure anything in the HV section while the oven is powered . If you must probe live. An isolation transformer is even limited value as well since the chassis IS the HV return and is a large very tempting place to touch. an isolation transformer can and should be used to test the primary side circuitry if necessary including interlocks. For example. prevent your scope probe ground from smoking should you accidentally connect an earth grounded scope to a live chassis.repairfaq. none of these devices will protect fools from themselves! Take extreme care whenever working with the cover off of a microwave oven. A circuit breaker is too slow and insensitive to provide any protection for you or in many cases. Actually. you will blow a fuse if you should forget to remove it when powering up the microwave. And. Don't attempt repair work when you are tired. Finally. triac/relay. motors.Notes on the Troubleshooting and Repair of Microwave Ovens
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terminals while working as added insurance. It would have to be HUGE due to the high power nature of a microwave oven and since the high voltage return is the chassis which is grounded. a GFCI (Ground Fault Circuit Interrupter) will NOT protect you from the high voltage since the secondary of the HV transformer is providing this current and any current drawn off of the secondary to ground will not be detected by the GFCI. use of a GFCI is desirable to minimize the risk of a shock from the line portions of the circuitry if you don't have an isolation transformer.deductive reasoning . Use an isolation transformer if there is any chance of contacting line connected circuits.) The use of a GFCI (Ground Fault Circuit Interrupter) protected outlet is a good idea but will not protect you from shock from many points in a line connected TV or monitor. or the high voltage side of a microwave oven. Isolation transformers and microwave ovens There's little point to using an isolation transformer with a microwave for testing the high voltage circuitry. for example. etc.

If you get stuck. Select a work area which is well lighted and where dropped parts can be located .it is both dangerous (particularly with microwave ovens) and mostly non-productive (or possibly destructive . Don't work when you are really tired . While not advocating being a pack rat. Old dead microwaves can often be valuable source of hardware and sometimes even components like interlock switches and magnetrons as these components are often interchangeable.very destructive). and plastic ice cube trays come in handy for sorting and storing screws and other small parts after disassembly. just letting the problem bounce around in your head will lead to a different more successful approach or solution.htm
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. The best location will also be relatively dust free and allow you to suspend your troubleshooting to eat or sleep or think without having to pile everything into a cardboard box for storage.
Test equipment
Don't start with the electronic test equipment. film canisters. Something like a large plastic tray with a slight lip may come in handy as it prevents small parts from rolling off of the work table. A basic set of high quality hand tools will be all you need to work on a microwave oven. A crimping tool will be needed as well but the $4 variety is fine for occasional use. Sometimes. Don't immediately assume that your problem is some combination of esoteric complex convoluted failures.org/sam/micfaq. An assortment of solderless connectors (lugs and wirenuts) is handy when repairing the internal wiring.Notes on the Troubleshooting and Repair of Microwave Ovens
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meter before power is applied and disconnect or move its probes only after power is removed AND the HV cap has been discharged (even if the meter catches fire or explodes!). there may be a defective door interlock switch or just a tired fuse. sleep on it. For a microwave oven. However. wire cutters and wire strippers. If you need to remove the cover or other disassembly. Many problems associated with consumer electronic equipment do not require a schematic (though one may be
http://www. These do not need to be really expensive but poor quality tools are worse than useless and can cause damage. See the document: Troubleshooting and Repair of Consumer Electronics Equipment for additional info on soldering and rework techniques and other general information. make notes of which screw went where . most of the power components in microwave ovens use solderless connectors (lugs) and replacements usually come with these as well. Needed tools include a selection of Philips and straight blade screwdrivers. A medium power soldering iron and rosin core solder (never never use acid core solder or the stuff for sweating copper pipes on electronic equipment) will be needed if you should need to disconnect any soldered wires (on purpose or by accident) or replace soldered components.not on a deep pile shag rug.they may not all be identical. start with some analytical thinking. Stanley or Craftsman are fine. needlenose pliers. More notes is better than less. Pill bottles. Qualified service people have been electrocuted using proper test equipment on microwave ovens!
Troubleshooting tips
Many problems have simple solutions.repairfaq. this does have its advantages at times.

htm
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. Unplug the oven. You will wonder how you ever lived without one! Cost: $25-50. Also see the sections: "Microwave leakage meters" and "Simple microwave leak detectors". Note: always have a load inside the oven when testing . Should you accidentally use the wrong test probe polarity with your meter.
Safe discharging of the high voltage capacitor
It is essential . This does not need to be expensive but since you will be depending on its readings. Even a relatively inexpensive DMM from Radio Shack will be fine for most repair work. however) and testing of interlock switches. Sometimes these homemade solutions do not survive for long but will definitely confirm that microwave power is present inside the oven chamber.org/sam/micfaq.
http://www. and only then change the connections. These are not super accurate or sensitive but are better than nothing. A DMM or VOM is necessary for checking of power supply voltages (NOT the high voltage. The technique I recommend is to use a high wattage resistor of about 5 to 50 ohms/V of the working voltage of the capacitor.Notes on the Troubleshooting and Repair of Microwave Ovens
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useful). There are special magnetron and microwave test instruments but unless you are in the business. reliability is important. A thermometer (glass not metal) to monitor water temperature during power tests.repairfaq. Other useful pieces of 'test equipment': A microwave leakage detector. These can be purchased or you can make one from a small neon (NE2) or incandescent bulb with its lead wires twisted together. The reason to use a large (high wattage) resistor is again not so much power dissipation as voltage holdoff. or continuity checks will identify most problems. Low voltage. WARNING: the high voltage in a microwave oven is NEGATIVE (-) with respect to the chassis. However. In any case. don't just interchange the probes = it may be last thing you ever do. these are unnecessary extravagances. High voltage probe (professional. You do not need an oscilloscope for microwave oven repair unless you end up trying to fix the logic in the controller .for your safety and to prevent damage to the device under test as well as your test equipment . fuses. several minutes may be required for the voltage to drop to negligible levels. You don't want the HV zapping across the terminals of the resistor. A microwave power detector. This will prevent the arc-welding associated with screwdriver discharge but will have a short enough time constant so that the capacitor will drop to a low voltage in at most a few seconds (dependent of course on the RC time constant and its original voltage). For the high voltage capacitor in a microwave oven. use a 25 W or larger 100 K ohm resistor for your discharge widget with a clip lead to the chassis. discharge the HV capacitor. this is only rarely actually required. The majority of microwave oven problems are easily solved with at most a multimeter (DMM or VOM). these can fail.that the large high voltage capacitor in the microwave generator be fully discharged before touching anything or making measurements. not homemade!). Even better is to use a proper high voltage resistor rated for at least 5 kV. resistance. While these are supposed to include internal bleeder resistors. and most of the components of the microwave generator. A series string of 10 to 20 1/2 W normal resistors in series can also be used. wiring. Inexpensive types are readily available at home centers or by mail order.extremely unlikely.a cup of water is adequate.

Safe Discharging and Other Related Information can be built into the discharge tool if desired. capacitors have been known to spontaneously regain some charge.including yourself. you may blow everything . It will not damage the capacitor (due to the current pulse).htm
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. Then. Don't just wrap it around .at least in principle. you will know that somehow. A microwave still under warranty should probably be returned for warranty service for any covered problems except those with the most obvious and easy solutions. 25W in this case) to a well insulated clip lead about 2 to 3 feet long. Again.org/sam/micfaq. It will not destroy screwdrivers and capacitor terminals.
Getting inside a microwave oven
You will void the warranty . You need to decide. A suitable discharge tool can be made as follows: Solder one end of the appropriate size resistor (100K ohms. it is a good idea to put a clip lead across the capacitor terminals just to be sure it stays fully discharged while you are working in the area. 2.1 second. Yes. this should drain the charge quickly and safely. Use the discharge probe on each side of the capacitor in turn for a second or two. There is a very slight chance the capacitor could be damaged by the uncontrolled discharge but at least there will be no danger. This discharge tool will keep you safely clear of the danger area. it is unlikely that this would be detected. If there is a big spark. Unplug the unit! Usually. always double check with a reliable high voltage meter or by shorting with an insulated screwdriver! Reasons to use a resistor and not a screwdriver to discharge capacitors: 1. It will reduce your spouse's stress level in not having to hear those scary snaps and crackles. 3.this connection must be secure for safety reasons.repairfaq. your original attempt was less than entirely successful. At worst. Finally. Solder the other end of the resistor to a well insulated contact point such as a 2 inch length of bare #14 copper wire mounted on the end of a 2 foot piece of PVC or Plexiglas rod which will act as an extension handle. The capacitor discharge indicator circuit described in the document: Capacitor Testing. If your discharging did not work. WARNING: DO NOT use a DMM for checking voltage on the capacitor unless you have a proper high voltage probe. confirm with a WELL INSULATED screwdriver across the capacitor terminals. There are usually no warranty seals on a microwave so unless you cause visible damage or mangle the screws or plastic. Secure the resistor to the insulating rod with some plastic electrical tape. you will blow the fuse upon powering up if you forget to remove it.Notes on the Troubleshooting and Repair of Microwave Ovens
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Clip the ground wire to an unpainted spot on the chassis. Since the time constant RC is about . the sheet metal cover over the top and sides is easily removed after
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So. They are not usually all the same! At least one of these includes a lockwasher to securely ground the cover to the case. Fortunately. they can make any sort of claim they want as to what might have been damaged even if all you did was remove and replace the cover without touching anything inside. everything may be really squeezed together. Most of these are on the back but a few may screw into the sides." Therefore. the oven lamp is actually inside the electronics bay right next to the high voltage on the magnetron filament .
Back to Microwave Oven Repair FAQ Table of Contents. If yours is a compact unit.org/sam/micfaq. :) Details will vary depending on manufacturer and model but most of the major components will look fairly similar to those depicted in the photo. Please see Typical Microwave Oven Electronics Bay for parts identification. the inside of a microwave is wide open and this is not difficult. Make sure ALL of the metal fingers around the front edge engage properly with the front panel lip.
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. nothing is powered inside (which is a good thing for safety!). How much of the controller is included varies but is usually minimal. particularly the one that grounds the cover to the chassis. The cover will then lift up and off. they will know someone has been inside.these are critical to ensure prevention of microwave leakage after reassembly.light bulb changing here is really best left to a professional if you would otherwise not go inside! Discharge the high voltage capacitor as described in the section: Safe discharging of the high voltage capacitor before even thinking about touching anything.
Principles of Operation
Instant (2 minutes on HIGH) microwave oven theory
Please see Typical Microwave Oven Electronics Bay for parts identification. "The repair will be $195 because you blew out the touch panel by removing the cover.repairfaq. and Premium Parts. Confirm that the screws you removed go back in the proper locations. Note that for this model. it is essential to make note of any differences in screw types so they can be put back in the same place. Kind of like Russian Roulette. there's a high probability that the oven no longer works at all. A schematic showing all of the power generation components is usually glued to the inside of the cover. But when the cover is replaced with the screws in random locations. Dalbani. If less than entirely honest. And. Not all ovens are this wide open. Take particular care to avoid pinching any wires when reinstalling the cover. This is critical to avoid microwave emissions should the waveguide or magnetron become physically damaged in any way. Fortunately.Notes on the Troubleshooting and Repair of Microwave Ovens
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unscrewing 8-16 philips head or hex head sheet metal screws. Note that on some ovens (I've heard that some Sharp models do this). all the parts in a microwave can be easily replaced and most of the parts for the microwave generator are readily available from places like MCM Electronics. there may also be one screw that is slightly longer than the others to engage a safety case interlock switch and prevent the oven from getting power if it is not present or one of the shorter screws is used in its place. if it's then taken to a service center. with the cover removed. Note how fingers on the cover interlock with the main cabinet . Reassemble in reverse order.

There is little transfer of energy directly to these materials. A microwave oven should never be operated without anything inside as the microwave generator then has no load . Thus plastic. Ignoring losses through convection. It is possible to heat a pure liquid like water to above its boiling point if there are no centers for bubbles to form such as dust specks or container imperfections. Heating is not (as popularly assumed) from the inside out. nearly all the energy generated by the oven is available to heat the food and heating speed is thus only dependent on the available power and how much food is being cooked. heating is via conduction from the burner or coil and there will be ample opportunity for small bubbles to form on the bottom long before the entire volume has reached the boiling point.45 GHz was probably chosen for a number of other reasons including not interfering with existing EM spectrum assignments and convenience in implementation. The penetration depth of the microwave energy is a few cm so that the outside is cooked faster than the inside. and so forth.45 GHz? Not that much. The 3 dB (half power) point is about 1 inch for liquid water .) "Industrial ovens still often operate at 915 MHz and other frequencies near 6 GHz are also used. the time to heat food is roughly proportional to its weight." Since the oven chamber cavity is a good reflector of microwaves. Water molecules are not resonant at this frequency. What is significant about 2. Microwave safe metal shelves will have nicely rounded corners. Water has numerous resonances over the entire spectra range.all the energy bounces around inside an a great deal is reflected back to the source. but the lowest frequency resonance is the rotational resonance is around 24 GHz. However. For references. With a stovetop. A wide range of frequencies will work to heat water efficiently.repairfaq. Other resonances occur in the millimeter wave range through the infrared. another 1/4 of the power in the next inch. check books on microwave spectroscopy by Townes and Gordy. glass. In addition.Notes on the Troubleshooting and Repair of Microwave Ovens
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A typical microwave oven uses between 500 and 1000 W of microwave energy at 2.org/sam/micfaq. This also means that the food does not need to be a conductor of electricity (try heating a cup of distilled water) and that electromagnetic induction (used elsewhere for high frequency non-contact heating) is not involved.half the power is absorbed in the outer 1 inch of depth. the interior filling will appear to be much hotter than the crust and will remain that way for a long time. One very real effect that may occur with liquids is superheating. 2. This can take place in a microwave since the heating is relatively uniform throughout the liquid. Since the pie can only cool from the outside. Ornitz (ornitz@tricon. The misconception may arise when sampling something like a pie filling just out of the microwave (or conventional oven for that matter). Thus two cups of water will take around twice as long to bring to a boil as one. From: Barry L. or even paper containers will heat only through conduction from the hot food. Such a superheated liquid may boil suddenly and violently upon removal from the oven with dangerous consequences.45 GHz to heat the food. Most metal objects should be excluded from a microwave oven as any sharp edges (areas of high electric field gradient) may create sparking or arcing which at the very least is a fire hazard. This heating is caused mainly by the vibration of the water molecules.net).htm
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.
http://www. unlike a conventional oven. the wavelength (about 5 inches) results in reasonable penetration of the microwave energy into the food. the microwave energy does penetrate these few cm rather than being totally applied to the exterior of the food.

The reason this works is that the wave that the dish picks up is longer than the hole in the mesh. The controller is what times the cooking by turning the microwave energy on and off. wire mesh (usually a sheet of metal which is either stamped or drilled with a hole pattern ..you want to catch all the possible sizes . (From: Filip (I'll buy a vowel) Gieszczykiewicz (filipg@repairfaq.htm
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.5cm "long" . and in high performance ovens. and applies this to a special type of vacuum tube called a magnetron . Did you ever see a "mesh" satellite disk up close? You will note that it looks much like it's made out of simple wire mesh that you can get in a hardware store (in the USA.
Why don't microwaves leak out from through the glass?
"I am trying to find out what the glass on a microwave consists of exactly.repairfaq. The microwave generator takes AC line power.without affecting the "watching the food" . and followed by a sheet of glass or plastic to make sure that food splatters and vapor condensation are easy to clean . The controller runs the digital clock and cook timer.yet still be able to see through it) they will not let anything out of the oven. it is essentially opaque to microwaves and essentially all the energy is reflected back into the oven cavity.. HIGH will be continuous on. if you can help. Consider bouncing a tennis ball on the "wire mesh" in the microwave . though very inexpensive units may simply have a mechanical timer (which ironically. Power level in most microwave ovens is set by pulse width control of the microwave generator usually with a cycle that lasts 10-30 seconds. i have not been able to get a better answer than 'a wire mesh'. It consists of two parts: the controller and the microwave generator. Since the holes in the mesh are much much smaller than the wavelength of the 2..45 GHz microwaves (about 5 inches or 12.
Controller
The controller usually includes a microcomputer. Power level is determined by the ratio of on time to off time in a 10-30 second cycle. like bouncing tennis and golf and ping-pong balls and marbles off the mesh . from the outside. BTW. it's not really "glass" but rather a 'sandwich' of glass.it WOULD work because the ball is bigger than the holes. runs the display. i would greatly appreciate it. you want them as small as possible . MEDIUM
http://www.org/sam/micfaq.) Greetings.little changed from its invention during World War II (for Radar).Notes on the Troubleshooting and Repair of Microwave Ovens
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This may cause expensive damage to the magnetron and other components. sets microwave power levels. The wave in the microwave is about 2.5 cm). For example.imagine scraping the mesh!
How a microwave oven works
The operation of a microwave oven is really very simple. monitors the moisture or temperature sensors.to minimize any stray and harmonic waves from escaping. A schematic diagram of the microwave generating circuitry and portions of the controller is usually glued to the inside of the cover. steps it up to a high voltage. as long as the holes are smaller than that (actually." There *is* a wire mesh embedded in the glass panel. it's called "chicken fence" :-).org)..like a color TV CRT mask!). is probably more expensive to manufacture!).

Typically has a secondary of around 2. system described below. some models use finer control. There will also be a low voltage winding for the Magnetron filament (3. The power ratios are not quite linear as there is a 1 to 3 second warmup period after microwave power is switched on. a pair of leads for the Magnetron filament. there have been some back in the 1970s that did this with a 1 second or so pulse width modulated cycle.org/sam/micfaq. Always inspect the cooling fan/motor for dust and dirt and lubricate if necessary. With a normal oven or stovetop. However. the air. and a single connection for the HV output.000 VRMS at 0.
Cooling fans
Since 30 to 50 percent of the power into a microwave oven is dissipated as heat in the Magnetron. wasted energy goes into heating the pot or oven. and LOW may be 5 seconds on. 15 seconds off. High Voltage Transformer. A couple of drops of electric motor oil or 3-in-One will go a long way. It consists of 5 parts: high voltage transformer. a defective overtemperature thermostat. There will be a pair of quick-connect terminals for the AC input. and so on. The controller activates the microwave generating circuitry using either a relay or triac. 10 seconds off. One interesting note: Since 30 to 50 percent of the power goes out the vents in the back as heat.repairfaq. If there are any belts. capacitor. capacitor. Most common are probes for temperature and moisture.
Microwave generator
This is the subsystem that converts AC line power into microwave energy. A convection oven will include a temperature sensor above the oven chamber. An oven that shuts off after a few minutes of operation could have a cooling problem. cooling is extremely important. this is relatively independent of the quantity of food and may be considered to be a fixed overhead. Since these sensors are exposed to the food or its vapors. waveguide to oven chamber. inspect for deterioration and replace if necessary. or is being operated from very high AC line voltage increasing power to the oven. there is a crossover point beyond which it is more efficient to use conventional heat than high tech microwaves. a microwave oven is really only more efficient than conventional means such as a stovetop or gas or electric oven for heating small quantities of anything. fast enough to have the same effect as continuous control for all practical purposes.25 amp more or less depending on the power rating of the oven.Notes on the Troubleshooting and Repair of Microwave Ovens
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may be 10 seconds on.htm
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. Therefore. However. However. a bad magnetron. The HV return will be fastened directly to the transformer frame and thus the chassis. These are typically "inverter" models which use a more sophisticated type of power supply than the simple high voltage transformer.3 V at 10 A is typical).
Sensors
More sophisticated ovens may include various sensors. The operating voltages for the controller usually are derived from a stepdown transformer.
http://www. You cannot miss this as it is the largest and heaviest component visible once the cover is removed. rectifier. rectifier diode. failures of the sensor probes themselves are common. even to the point of a continuous range of power. magnetron.

Capacitor . the filament/HV connections on the bottom section. it's not enough to provide any reduction in the likelihood of electrocution should you come in contact with the HV winding! Rectifier .basic textbook description
http://www.. Here are two descriptions of magnetron construction. The first is what you will likely find if you go to a library and read about radar. Note that this use of 'working voltage' may be deceiving as the actual voltage on the capacitor may exceed this value during operation. (Some really old microwave ovens may use the classic design as well. was standing near one of the high power radar units and noticed that a candy bar in his shirt pocket had softened. Most commonly.htm
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. Dr. Magnetron . and output antenna.65 to 1. a researcher at the Raytheon Corporation.2 uF at a working voltage of around 2.45 GHz. Percy Spencer. However.no disassembly required!) So there would about 3 turns for the magnetron filament and 2080 turns for the high voltage winding for the transformer mentioned above.
Magnetron construction and operation
The cavity magnetron was invented by the British before World War II.usually rated 12. it is cylindrical in shape but this is less common. (It's usually possible to count the primary turns by examining how it is wound . (Items (1) to (6) in the following sections apply to each type while items (7) to (9) apply to both types.000 to 15. see the articles at the Microtech Web Site. he decided to investigate further.5 amp. In the typical 'I have to know why this happened' mentality of a true scientist.) This is followed by my autopsy of a dead magnetron of the type that is probably in the microwave oven in your kitchen. Magnetron construction . Topics include basic microwave theory as well as a complete discussion of microwave oven magnetron construction and principles of operation. possibly to compensate for various magnetron load conditions. The reason they can get away with so few turns is that it operates fully loaded about 90 percent of the time but is still on the hairy edge of core saturation.) For more detailed information with some nice diagrams. There is also generally a "magnetic shunt" in the core of the transformer. it is a box bolted to the chassis. The story goes that shortly after the War. Sometimes. multiple resonant cavities with a pair of permanent ceramic ring magnets to force the electron beams into helical orbits.5 inch long with wire leads. The capacitor is metal cased with quick-connect terminals on top (one end). It is considered by many to be the invention most critical to the Allied victory in Europe. this will be rectangular or cylindrical.000 PRV at around . Sometimes. One end will be electrically connected to the chassis. The magnetron is most often box shaped with cooling fins in its midsection.repairfaq. Always discharge the capacitor as described below before touching anything inside once the cover is removed. The Amana Radarange and the entire future microwave oven industry were the result.Notes on the Troubleshooting and Repair of Microwave Ovens
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These transformers are designed with as little copper as possible.the microwave producing tube includes a heated filament cathode. The frequency of the microwaves is usually 2.thus about 1 turn per volt input and output (this is about 1/4th as many turns as in a "normal" power transformer.org/sam/micfaq. This provides some current limiting. The primary for 115 VAC is typically only 120 turns of thick wire . and the antenna (hidden by the waveguide) on top. about .000 VAC.

a dozen or so turns of heavy wire on a ferrite core . The filament gets its power via a pair of high current RF chokes . 5/32" (4 mm) diameter and just over 3/8" (9. A centrally located cylindrical electron emitting cathode. The filament and cathode are one in the same and made of solid tungsten wire. A cylindrical anode block surrounding but separate and well insulated from the cathode. The cathode is coated with a material which is good for electron emission. or oil since the microwave generation process is only about 60 to 75 percent efficient and these are often high power tubes (many kilowatts).45 GHz (12. 2.5 mm) diameter. (For the frequency of 2. Multiple cylindrical resonator cavities at a fixed radius from the cathode bored in the anode block. This is supplied with pulsed or continuous power of many thousands of volts (negative with respect to the anode. This is a view looking up through the anode cylinder from the filament end of the tube. water.5 mm) in length.94 times the diameter of the cavities.to prevent microwave leakage back into the filament circuit and electronics bay of the oven. However.3 VAC at 10 A. Note: this coating is the only material contained in the microwave oven magnetron that might be at all hazardous. The original Amana Radarange and other early microwave ovens likely used this design as well. The entire assembly is placed in a powerful magnetic field (several thousand Gauss compared to the Earth's magnetic field of about . 1. may be used in large radar magnetrons but should not be present in the types found in domestic microwave ovens.Notes on the Troubleshooting and Repair of Microwave Ovens
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This is the description you will find in any textbook on radar or microwave engineering.org/sam/micfaq. 1. See the text below for parts names and dimensions. a toxic metal. This is usually supplied by a permanent magnet though electromagnets have been also used. 3. formed in a helix with about 8 to 12 turns. about .repairfaq. Also see this photo of the Typical Magnetron Anode and Resonant Structure. The item numbers are referenced to the diagram in the section: Cross section diagram of typical magnetron.000 V. The original designs used huge somewhat horseshoe shaped permanent magnets which were among the most powerful of the day.7 mm).4 cm) used in a microwave oven this would result in a cavity diameter of approximately . 6.htm
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. 5. An antenna pickup in one of the cylindrical cavities which couples the microwave energy to the waveguide. The wavelength of the microwave energy is approximately 7. Typical filament power is 3. Beryllium. Channels link the cavities to the central area in which the cathode is located. nearly all other magnetrons used in modern domestic microwave ovens should be very similar. The cathode is supplied with a pulsating negative voltage with a peak value of up to 5.
http://www.020" (.modern microwave oven This description is specifically for the 2M214 (which I disassembled) or similar types used in the majority of medium-to-high power units. Cooling of the anode block must be provided by forced air.62" (15. 4.5 Gauss). Magnetron construction .

3.5 mm) thick and approximately 1/2" (12. exits the tube via a cylindrical ceramic insulator sealed to the top cover. When powered. 8.5 mm) thick copper with an inside diameter of 13/8" (35 mm) and a length of about 1" (25.5 Gauss).htm
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. all the even numbered vanes are shorted to each other and all the odd numbered vanes are shorted to each other. individual inductors and capacitors are not used. Rather than cylindrical cavities (as you would find in most descriptions of radar magnetrons). This structure results in multiple resonant cavities which behave like sets of very high quality low loss L-C tuned circuits with a sharp peak at 2.4 mm). For the 2M214. These are brazed or silver soldered to the inside wall of the cylinder facing inward leaving a 5/16" (8 mm) central area clear for the filament/cathode. There will always be a cooling fan to blow air through this assembly.org/sam/micfaq.5 mm) wide. these are about 2-1/8" (54 mm) OD. At this high frequency.062" (1. 7. The simplest way to describe what happens is that the electron bunches brush against the openings of the resonating cavities in the anode and excite microwave production in a way analogous to what happens
http://www.7 mm) long by 3/8" (9. and the resonant cavities. Of course. The entire assembly is placed in a powerful magnetic field (several thousand Gauss compared to the Earth's magnetic field of about . 5.7 mm) thick. 1-13/16" (46 mm) ID. and anode cylinder. and attaches to the pressed-on bu?ll-nose antenna cap.5 mm) thick edges of the 10 vanes with gaps of approximately . Thus.Notes on the Troubleshooting and Repair of Microwave Ovens
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2. 6. Steel plates (which probably help to shape the magnetic field. A set of thin aluminum fins act as a heat sink for removing the significant amount of wasted heat produced by the microwave generation process since it is only about 60 to 75 percent efficient. there are a set of 10 copper vanes . Surrounding this space are the . Copper shorting rings at both ends near the center join alternating vanes. The magnetic field forces them to travel in curved paths in bunches like the spokes of a wheel. Magnetron construction .repairfaq. It passes through a hole in the top end plate.45 GHz. The anode and magnetron case are at ground potential and connected to the chassis. are all in a vacuum. This is provided by a pair of ceramic ring magnets placed against the top and bottom covers of the anode cylinder. 4. The anode is a cylinder made from . The filament leads/supports enter through a cylindrical ceramic insulator sealed to the bottom cover and then pass through a hole in the bottom end plate.062" (1.062" (1. These are press fit on the magnetron anode and also in contact with the magnetron case. 1/2" (12. see below) and thin steel covers (to which the filament and antenna insulators are sealed) are welded to the ends of the cylinder. The gap between the cathode and anode.common features The following items apply to all types of magnetrons. electrons stream from the cathode to the anode. all the rings are also all shorted at the outside where they are joined to the inner wall of the cylinder.04" (1 mm) between them. A connection is made near the middle of a single vane to act as the output power takeoff. shorting rings. The inductance and capacitance are provided by the precise configuration and spacing of the copper vanes.

WARNING: What this implies is that if the magnetron is not present or is not drawing power for some reason . The magnetron load.Notes on the Troubleshooting and Repair of Microwave Ovens
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|| +------------------------+ ||( 3. it probably doesn't matter which way they are connected. Other types of power supplies have been used in a few models .the magnetron is across the diode. At the end of normal operation.Chassis ground
Note the unusual circuit configuration . The bleeder may be defective and open as this does not effect operation of oven and/or the time constant may be long . Then. The output waveform looks like a sinusoid with a p-p voltage equal to the p-p voltage of the transformer secondary with its positive peaks at chassis ground (no load). 10 A.repairfaq.like an open filament ._---+---/ -. not the capacitor as in a 'normal' power supply.414 where V(RMS) is the output of the high voltage transformer. and reliability of the half wave doubler configuration.thermal protector . There is also usually a bleeder resistor as part of the capacitor. The voltage across the HV rectifier will then be: V(peak) + V where V is the waveform out of the transformer.including high frequency inverters but it is hard to beat the simplicity.somewhere in the primary circuit.minutes. What this means is that the peak voltage across the magnetron is the transformer secondary + the voltage across the capacitor.which will be approximately V(peak) = V(RMS) * 1. The peaks are negative with respect to the chassis.25 A | HV |'--> Micro||( typical | Diode | waves (Controller not shown) || +------------+---------+ _|_ . while F goes to the lone Filament terminal on the HV transformer.----/ ----+ || +------||----+ | |_ _| | | )||( HV Cap | | \/ | AC I \ I=Interlock )||( __|__ | ___ | Line | TP=Thermal Prot. In addition. However. Functionally. This is a half wave voltage doubler.htm
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.up to V(peak) will still be present across the capacitor when power is removed. low cost. Some ovens may not have a bleeder at all.up to 5000 V at AMPs available! WARNING: Never attempt to view this waveform on an oscilloscope unless you have a commercial high voltage probe and know how to use it safely! The easiest way to analyze the half wave doubler operation is with the magnetron (temporarily) removed from the circuit. Note that there is a difference in the labels on the filament connections of the magnetron.org/sam/micfaq. The negative peaks will get squashed somewhat under load. There may also be a thermal fuse or other
http://www. often bolted to the magnetron case. there will likely be an over-temperature thermostat .ALWAYS DO THIS IF YOU NEED TO TOUCH ANYTHING IN THE MICROWAVE GENERATOR AFTER THE OVEN HAS BEEN POWERED. so the peaks will approach the peak-peak value of the transformer or nearly 5000 V in the example above.000 V due to the load since the magnetron does not conduct at low voltages. reduces the peak value of this somewhat .where most of its conduction takes place. )||( 2. the typical schematic (as above) shows FA going to the node attached to the Anode of the HV diode. being across the HV diode. it becomes a simple half wave rectifier/filter so far as the voltage acrtoss the capacitor is concerned . not shown. some of this will likely be discharged immediately but will not likely go below about 2. See the section: High frequency inverter type HV power supplies. typical | TP Relay or || +------------+------+FA F| Magnetron _ Fuse I __ Triac || | +-|----|-+ o------.3 VAC.000 VAC _\_/_ +----|:--+ o------------+-------------------+ ||( . HOWEVER: DO NOT ASSUME THAT THIS IS SUFFICIENT TO DISCHARGE THE CAPACITOR . Take extreme care .

The interlocks must be activated in the correct sequence when the door is closed or opened. (That interlock may be known as a "dummy switch" for obvious reasons and is often not even mentioned in the schematic/parts manifest. etc. This makes it more difficult for an ignorant consumer to just bypass the door interlocks should they fail or to run the oven with an open door as a room heater . At least one of these will be directly in series with the transformer primary so that a short in the relay or triac cannot accidentally turn on the microwaves with the door open. The typical door switches and their function: Door Sensing: Input to the microcontroller to indicate the state of the door.
Troubleshooting Guide
Instant troubleshooting chart .Notes on the Troubleshooting and Repair of Microwave Ovens
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protector physically elsewhere but in series with the primary to the high voltage transformer. Otherwise.org/sam/micfaq. Other parts of the switched primary circuit include the oven interlock switches. Complete meltdowns are not unusual! If any defective door switches are found. And.
Back to Microwave Oven Repair FAQ Table of Contents. The interlocks are designed so that if the door is correctly aligned. they will sequence correctly. cooling fan.htm
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. turntable motor (if any).and protects the manufacturer from lawsuits. oven light. peculiar behavior may occur (like the fan or turntable operating at the wrong time) but should never result in microwaves being generated with the door open. another interlock is set up to directly short the power line if it is activated in an incorrect sequence.) Of course.repairfaq. but the switch contacts will likely be damaged by the high initial current! This also means it probably wouldn't be a bad idea to replace the interlock switch which might have been affected if your oven fails with a blown fuse due to a door problem.perhaps as high as 75 percent. opening the door to interrupt a cook cycle results in arcing at the contacts. Primary Interlock: In series with the high voltage (magnetron) power supply so cuts power when the door is open. Interlock Monitor: Shorts out the AC line (and blows the main fuse) should the Primary Interlock not open due to incorrect sequencing of the door switches or a failed switch. should that switch ever actually be used. Note that if the Door Sensing switch should malfunction. Failed door interlocks account for the majority of microwave oven problems . Interestingly. This is not surprising considering that two of the three switches carry the full oven current any deterioration of the contacts results in increased resistance leading to their heating and further deterioration. a short will be put across the power line causing the fuse to blow forcing the oven to be serviced. it is probably a good idea to replace all of them as long as the oven is already apart.
Interlock switches
Various door interlock switches prevent inadvertent generation of microwaves unless the door is closed completely.most common problems and possible causes
http://www. not only will the fuse blow.

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The following chart lists a variety of common problems and nearly all possible causes. Diagnostic procedures will then be needed to determine which actually apply. The 'possible causes' are listed in *approximate* order of likelihood. Most of these problems are covered in more detail elsewhere in this document. While this chart lists many problems, it is does not cover everything that can go wrong. However, it can be a starting point for guiding your thinking in the proper direction. Even if not listed here, your particular problem may still be dealt with elsewhere in this document. Problem: Totally dead oven. Possible causes: 1. 2. 3. 4. 5. No power to outlet (blown fuse or tripped breaker or GFCI). Blown main fuse - likely due to other problems. Open thermal protector or thermal fuse. Defective controller or its power supply. Clock needs to be set before other functions will operate (some models).

Problem: Totally dead oven after repair. Possible causes: 1. Cabinet screws replaced in incorrect location (safety interlock not engaged). 2. Any number of screwups. :) Problem: No response to any buttons on touchpad. Possible causes: 1. 2. 3. 4. 5. 6. 7. Door is not closed (some models). You waited to long (open and close door to wake it up). Controller is confused (pull plug for a minute or two to reset). Defective interlock switches. Faulty controller or its power supply. Touchpad or controller board contaminated by overenthusiastic cleaning. Defective/damaged touchpad.

Problem: Oven runs when door is still open. Possible causes: 1. Damaged interlock assembly. 2. Cooling fans (only) running due to bad sensor or still warm. Problem: Oven starts on its own as soon as door is closed. Possible causes: 1. 2. 3. 4. 5. Defective triac or relay. Controller is confused (pull plug for a minute or two to reset). Defective controller or its power supply. Touchpad or controller board contaminated by overenthusiastic cleaning. Defective/damaged touchpad.

Problem: Oven works but display is blank. Possible causes: 1. Defective controller or its power supply.

Problem: Some keys on the touchpad do not function or perform the wrong action. Possible causes: 1. 2. 3. 4. Touchpad or controller board contaminated by overenthusiastic cleaning. Defective/damaged touchpad. Controller is confused (pull plug for a minute or two to reset). Faulty controller.

What can go wrong
The most common problems occur in the microwave generating portion of the system, though the controller can be blown by a lightning strike or other power surge. Bad interlock switches probably account for the majority of microwave oven problems. Also, since the touchpad is exposed, there is a chance that it can get wet or damaged. If wet, a week or so of non-use may cure keys that don't work. If damaged, it will probably need to be replaced - this is straightforward if the part can be obtained, usually direct from the manufacturer. Unfortunately, it is an expensive part ($20-50 typical). The interlock switches, being electromechanical can fail to complete the primary circuit on an oven

http://www.repairfaq.org/sam/micfaq.htm

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repairfaq. a relative. It is warm. See the section: Testing and replacing of interlock switches.
Totally dead oven
First. garage sale. cozy. or friend.org/sam/micfaq. and from their point of view makes an ideal habitat. Clean the circuit board and connectors thoroughly with water and then isopropyl alcohol. the microcontroller will get into a whacko mode for some unknown reason .htm
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. First. No adjustments should ever be required for a microwave oven and there are no screws to turn so don't look for any!
General system problems
The following problems are likely power or controller related and not in the microwave generator unless due to a blown fuse or bad/intermittent connections: Totally dead oven. Creatures with six or more legs (well. Note: when working on controller related problems. Dry completely. or if your kitchen isn't the cleanest in the world.it is often a separate connector. some two legged varieties as well) are not known for their skills in the areas of housekeeping and personal hygiene. unplug the connection to the microwave generator (HV transformer primary) from the power relay or triac . no electronic components were affected though there is always a slight possibility of other problems.and simply needs to be reset. Failed interlocks are considered to be the most common problems with microwave ovens. Sometimes. check power to the outlet using a lamp or radio you know works. Whacked out controller or incorrect operation. No response to any buttons on touchpad Oven runs when door is still open. Faulty interlocks or a misaligned door may result in the fuse blowing as described above due to the incorrect sequencing of the door interlock switches. Oven works but display is blank. Some keys on the touchpad do not function or perform the wrong action. Inspect the circuit traces for corrosion or other damage. Oven starts on its own as soon as door is closed.Notes on the Troubleshooting and Repair of Microwave Ovens
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which appears to operate normally with no blown fuses but no heat as well. This will prevent any possible accidental generation of microwave energy as well as eliminating the high voltage (but not the AC line) shock hazard during servicing. Hopefully. the curb. If you got the microwave oven from a flea market. The fuse or circuit breaker at
http://www. perhaps as high as 75% of all failures. The problem may never reoccur. these will have be be jumpered with fine wire and then soldered. safe. unplug the microwave oven for a couple of minutes. there is likely a problem with the controller circuitry or its power and you will have to get inside the oven.perhaps a power surge . If there are any actual breaks. Erratic behavior. If this does not help.
Uninvited guests
Some cockroaches (or other lower life forms) may have taken up residence on the controller circuit board. Microwave oven does not respond to START button. such visitors are quite possible.

Totally dead oven after repair
On some microwave ovens.microwave ovens are high current appliances and should be on a dedicated circuit if possible. Other possible causes: bad controller power supply or bad controller chip.remove and inspect the light bulb and socket. Furthermore. If this blows immediately. Or.there may be nothing wrong with it. A refrigerator should never be plugged into the same circuit for this reason as well you really don't want it to be without power because of your popcorn! If you find the fuse blown or circuit breaker tripped. try to set the clock. power relay. If this is the case. in the controller. Next. and/or triac from the controller. Some models have a thermal fuse as well and this may have failed for no reason or a cooling fan may not be working and the oven overheated (in which case it probably would have died while you were cooking something for an important guest . a fuse has probably blown although a dead controller is a possibility.then one of the components in the microwave generator is defective (shorted). This is common. Replace the fuse or reset the circuit breaker. Assuming these are not your problems. You may just have too many appliances plugged into this circuit . This engages a safety interlock which prevents the oven from receiving power if the correct screw is missing or in the wrong hole. Therefore. then putting in a new fuse will enable the touchpad/display to function but may blow again as soon as a cook cycle is initiated if there is an actual fault in the microwave circuits. there is a short circuit in the power cord or elsewhere. unplug everything from the circuit to which the microwave is connected (keep in mind that other outlets may be fed from the same circuit). even a shorted oven lamp . there is at least one cabinet screw that is slightly longer than all the others.htm
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.Notes on the Troubleshooting and Repair of Microwave Ovens
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your service panel may have blown/tripped due to an overload or fault in the microwave oven or some other appliance. See the section: Microwave generator problems. If the oven now works.repairfaq. If the main fuse is upstream of the controller. there may be a short very near the line cord. some ovens will not allow you perform any cooking related actions until the clock is set to a valid time.org/sam/micfaq.and it appears to operate normally .assuming you would use a microwave oven for such a thing!). confirm that the controller is operational by unplugging the microwave generator. you *will* blow the fuse or trip the circuit breaker. If the fuse still blows immediately. Check the length of all the screws and locate the interlock
http://www.) The GFCI outlet may not be in an obvious location but first check the countertop outlets. (Removing a broken oven lamp has been known to happen. With some ovens the screen will be totally blank following a power outage . The tripped GFCI could be in the garage or almost anywhere else! Pushing the RESET button may be all that's needed. The microwave oven may be powered from a GFCI outlet or downstream of one and the GFCI may have tripped. try a new fuse. If the same thing happens again. If it does not blow. you have a problem with the outlet or other wiring on the same branch circuit. initiate a cook cycle (with a cup of water inside). If a new fuse does not now blow when a cook cycle is initiated . If you attempt to run a heating appliance like a toaster or fryer at the same time. If plugging in the microwave causes the fuse to blow or circuit breaker to trip immediately. the fuse may simply have been tired of living. or a defective triac (if your oven uses a triac). then any short circuit in the microwave generator will also disable the controller and display.

the transformer will need to be replaced.
No response to any buttons on touchpad
There can be many causes for this behavior (or lack of behavior): Door is not closed . I don't know how common this practice is but have heard of it on some Sharp models. There is a good chance that the surge didn't propagate beyond the transformer and thus the rest of the controlled should be unaffected. As always. Faulty controller or its power supply . then check the power supply for the controller next. Check the primary of the power transformer . Some of these thin traces may be there specifically to act as fuses .and there may even be spares to use for just this situation! Assuming that the main fuse and power transformer primary checks out.this can result in the controller thinking the door is open and ignoring you. You waited too long .htm
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.unless the door is securely closed.org/sam/micfaq.Notes on the Troubleshooting and Repair of Microwave Ovens
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switch behind one of the screw holes. If you close the door but don't proceed to activate any functions with a couple of minutes. diagnosing this will be tough without a schematic (and possibly much more). any number of other pre-existing or induced problems can result in the oven playing dead after it has been "repaired". circuit board traces may have been vaporized (but repair may still be possible by simply jumpering across the crater). also check for bad solder connections. At this point. If not. a schematic of the controller board will be needed .on many ovens. :
Dead controller
The most common way that the controller circuitry can be harmed is by a power surge such as from a lightning strike.a power surge may have damaged the electronics. Other than checking for bad connections and obviously bad power supply components. there may be a fuse/thermal fuse underits outer insulation.repairfaq. In some cases. they will require you to open and close the door to reset their pathetic brains. Pull the plug for a minute or two to reset it.and replacement controller or even just the main chip may be nearly as expensive as a complete new oven. only components on the primary side of the power transformer will be affected. It could be a simple part like a capacitor or diode.some models (like Sharp) have a timeout. Of course. Also see the section: Getting inside a microwave oven.even setting the clock . Defective interlock switches .
http://www. Hopefully. there will be no response to any buttons . but they would all need to be tested.a power surge or random non-reproducible action of the universe may have resulted in the controller's program ending up in an infinite loop. If the controller power supply is working and there is still no sign of life (dead display and no response to buttons) the microcontroller chip or some other part may be bad. Controller is confused .if it is open.often impossible to get .

Defective or damage touchpad .for some reason the display portion of the controller may have been sent out to lunch by a power surge or alpha particle. put a cup of water into the oven and let it run for a minute to check for heating. a failure of this type is extremely unlikely since power to the microwave generator passes through 2 of the 3 interlock switches. Also see the section: Some of the keys on the touchpad do not function or perform the wrong action. the controller would think the door was always closed.
http://www. First. unplug the oven for a couple of minutes to try to reset the controller. the problem is almost certainly in the controller or its power supply.
Oven runs when door is still open
WARNING: Needless to say. If both of these failed in the closed position.regardless of whether a cook cycle has been selected. It woudn't be the first time.org/sam/micfaq. some may have gotten inside and shorted out the touchpad or controller. Try pulling the plug for a minute or two .the outer film is broken .
Oven starts on its own as soon as door is closed
If the oven starts up as soon as the door is closed . Also see the section: Whacked out controller or incorrect operation. the third switch would have blown the fuse the last time the door was opened. DO NOT operate the oven with the door open! While extremely unlikely. Where no such damage is evident. (You could also note the normal sound change or slight dimming of lights that accompanies operation of the magnetron. Another more benign possibility is that one or more fans are running as a result of either a defective sensor or normal operation to maintain air flow until all parts have cooled off. If this doesn't help.physical abuse is not a recommended technique for getting a microwave oven to cooperate.repairfaq. the microwave be generator could be running! For microwaves to actually be generated with the door still open would require the failure of all 3 interlock switches.htm
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.if you recently power washed the oven (or even if you only use some spray cleaner).
Oven works but totally dead display
If all functions work normally including heating but the display is blank (assuming you can issue them without being able to see the display).it will probably need to be replaced. If there is any visible damage to the touchpad . The only way this could really happen would be for the 'fingers' from the door that engage the interlocks to break off inside the oven keeping the interlocks engaged.) Much more must be enabled to actually power the magnetron so this might point more to the controller as being faulty but not always. In this case.Notes on the Troubleshooting and Repair of Microwave Ovens
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Touchpad or controller board contaminated by overenthusiastic cleaning . the cause could be a shorted triac or relay or a problem with the controller or touchpad. Check for bad connections between the display panel and the power supply and solder joints on the controller board.

A controller failure does little to predict the reliability of the rest of the oven. or be displaying in Greek. My guess is that unless you were to find some simple bad connections or an obvious problem with the controller's power supply.htm
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.possibly just a power supply but could also be the controller chip. unless this oven has a lot of fancy features. In this case. First.open and close the door to reset. some or all operations . more serious damage to the door seals may have resulted as well which would be a definite hazard. see the section: Erratic behavior. The output of the magnetron tube may decrease slightly with use but there is no particular reason to expect it to fail any time soon.
Whacked out controller or incorrect operation
The following are some of the possible symptoms: All the display digits may have come on. Depending on the model.even setting the clock . are you sure a previous cook cycle was not interrupted and forgotten? Try to recreate the problem using a cup of water as a load. the display panel may have fractured though it would take quite a bit of violence. for the special case where pressing START results in erratic behavios. some liquid may have accidentally gotten inside the touchpad or even the controller circuitry (though this is less likely). See the section: Some of the keys on the touchpad do not function or perform the wrong action. If the oven seems to have a mind of its own .repairfaq. (This could also be a shorted triac or power relay). The microwave generator circuits could last a long time or fail tomorrow. This and the other parts are easily replaceable. so these should be checked. Assuming this does not apply. (By 'tone' I mean from the controller (not a low buzzing or humming when attempting to cook which would indicate a microwave generator power problem like a shorted magnetron). the cost to repair would be very high as the custom parts are likely only available from the manufacturer.may be conditional on the door interlocks being closed. try unplugging the oven for a couple of minutes . Pressing a button on the touchpad may result in a totally incorrect action such as entering the time resulting in the oven starting to cook. If you recently cleaned the oven. a bad microcontroller chip is not that likely but is still a possibility. EEEE or FFFF.perhaps the controller is just confused due to a power surge. If the oven was physically abused. it sounds like a controller problem . The controller's program may be corrupted (unlikely) but we have no real way of diagnosing this except by exclusion of all other possibilities. Pressing buttons on the touchpad may or may not have any effect.
http://www.Notes on the Troubleshooting and Repair of Microwave Ovens
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With everything else operational. Some ovens will not allow any actions to be performed if the door has been closed for more than a few minutes . lightning strike or the EMP from a nearby nuclear detonation because it wanted attention. The end-of-cooking cycle or keypress tone may be wailing away continuously. However.running a cycle you didn't think you programmed. However. you can buy a replacement (depending on size) for $100-200 so it is probably not worth fixing unless it is something relatively simple and inexpensive. The oven may start cooking (or at least appear to) as soon as the door is closed.org/sam/micfaq.

suspect a power relay or mechanical timer (if used) with dirty or worn contacts. the timing motor could be defective or require lubrication. For mechanical timers. Reseat the flex cable connector to the touchpad. There may be bad connections or loose lugs. a thorough search for loose ground and other connections and bad solder joints may locate the source of the difficulty. there are several possibilies: The power surge caused by the cook cycle starting is resulting in changes to the settings or else the microcontroller is not interpreting them properly. Prod the logic board to see if the problem comes and goes. As with intermittent problems.htm
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. The surge or vibration of starting can jiggle the element open or closed. If you suspect either of these. see the section: Oven heats but power seems low or erratic. Unfortunately. this is sometimes difficult to pinpoint because unless there is obvious mechanical damage. inspect the integrety of the magnetronwaveguide joint and make sure the RF gasket is in place. Microwave (RF) leakage into the electronics bay due to an faulty joint between the magnetron and the waveguide or structure failure of the magnetron may be interfering with the operation of the microcontroller. the main fuse may become intermittent rather than failing completely. or (less likely) the power surge from energizing the microwave generator or microwave (RF) leakage into the electronics bay affecting the controller. This may be due to a faulty part of bad connections in the controller or elsewhere. the 'problem' may disappear once the cover is removed for testing. It is easy to try replacing it!
Problems with internal microwave leakage
(From: Charles Godard (cgodard@iamerica. If the times and power levels appear on the display reliably but then become scrambled when entering the cook cycle or the oven behaves strangely in some other way when entering the cook cycle. Unless the oven was dropped or 'repaired' by an butcher. if erratic simply means that it doesn't heat consistently. See the section: Problems with internal microwave leakage. However.Notes on the Troubleshooting and Repair of Microwave Ovens
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Erratic behavior
There are three different situation: Whenever the oven performs unexpectedly both during setup and the cook cycle. If the oven uses a HV relay for power control.org/sam/micfaq.repairfaq. The contacts could be dirty or worn. On rare occasions. The primary relay may have dirty or burnt contacts resulting in erratic operation.) I only service Amana's. Where problems only occur when entering or during the cook cycle. Check with a capacitor meter or substitute known good ones. but have serviced lot's of them over the years. The filter capacitor(s) in the controller's power supply may be dried up or faulty. suspect the controller power supply or bad connections. this sort of failure is unlikely.net). this may be defective. I've only found a few that
http://www.

use resistors to jumper the proper contacts on the flex cable connector to simulate key presses. for example). when I was messing with the Controller PCB.htm
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. Then. After that I shelled out the bucks and bought a real detector. It turned out that the radiation was affecting the controller. Also. Our calibration meter showed a two watt leakage. I am skeptical as this could just be a side effect of a bad connection elsewhere.Notes on the Troubleshooting and Repair of Microwave Ovens
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leaked with my expensive leak detector. However. check the snap type connector where the touchpad flex-cable plugs into the controller board. or for no reason at all. the internal circuitry of the touchpad can be determined. Look carefully for any visible signs of damage or spills. of course dropping off to near zero a few inches away. (From: Matthew Sekulic (goatboy@telusplanet. Presumably. and it detected leaks on everything. From the outside. Reseating this cable may cur a some keys dead problem. similar symptoms. I checked for radiation with the cover off the unit and found it extremely high. (My worst case of actual external leakage was from a misaligned door at . Some people have reported at least temporary improvement by simple peeling the touch pad off of the front panel and flexing it back and forth a few times. The symptom was that the Touch Pad timer lights and indicators would change while the unit was cooking.org/sam/micfaq. any damage or just old age may permit spilled liquid to enter and short the sensors. The touchpads often use pressure sensitive resistive elements which are supposed to be sealed.repairfaq.) I have had a similar experience with a Sanyo. My clue in was a spark between the waveguide and the case. I took it apart and checked for loose solder joints and even cleaned the glass touch pad contacts. usually only available from the original manufacturer.75 watts with the probe's styrofoam spacer placed against the door. this dislodges some bit of contamination. with none escaping the outer case when attached. Long ago. This should permit the functions to be verified before a new touchpad is ordered. the unit didn't leak. For some reason that I don't remember now. I just reformed the gasket and reseated the magnetron and that fixed the leak. but with the leakage from the spot welded waveguide inside the unit. Caution: unplug the microwave generator from the controller when doing this sort of experiment!
http://www.)
Some of the keys on the touchpad do not function or perform the wrong action
Touchpads are normally quite reliable in the grand scheme of things but can fail as a result of physical damage (your spouse threw the roast at the oven). liquid contamination (from overzealous cleaning. This may require peeling it off of the front panel).net). With a little bit of effort (or perhaps a lot of effort). it may be necessary to replace the touchpad unit. I thought I had a timer problem. A week or so of drying may cure these problems. with the cover on. The most memorable was the one with the leak that was due to the copper gasket that's between the magnetron tube and the cavity. If there is actual visible damage. I tried one of the cheapie detectors because one of my parts supply houses suggested it.

(However. Otherwise.repairfaq. if the
http://www. then there may be a bad interlock or some other problem with the controller. Most of these are easy to diagnose and the required parts are readily available at reasonable prices. after all. Use this to confirm the basic controller logic and interlock circuitry. remove the touchpad entirely and wash it in clean water in an effort to clear out any contamination. pressing START does nothing. try it. cook time.perhaps it thinks an interlock is open.
No heat but otherwise normal operation
If the main power fuse is located in the primary of the high voltage transformer rather then at the line input. Oven immediately starts to cook when door is closed. Loud hum and/or burning smell when attempting to cook. the resistance should go down dramatically (similar to the other buttons). Check that there are no missing power supply voltages for the controller and bad connection. sees quite a lot of action! Assuming it is not the touch pad.
Microwave generator problems
Failures in the microwave generator can cause various symptoms including: No heat but otherwise normal operations. if there is a momentary response but then the oven shuts off.org/sam/micfaq. including no relay action and the timer digits do not count down.
Microwave oven does not respond to START button
While all other functions operate normally including clock. Fuse blows when closing or opening door. the clock and touchpad will work but the fuse will blow upon initiating a cook cycle. Arcing in or above oven chamber. Oven heats but power seems low or erratic. Fuse blows when initiating cook cycle. then do the same using high purity alcohol to drive out the water. It is as though the START button is being totally ignored. Check for bad interlocks or interlocks that are not being properly activated. See the section: Some of the keys on the touchpad do not function or perform the wrong action. If there is an alternate way of activating the cook cycle. However. Or. If it is also ignored. If it works. then the problem may indeed be a faulty START button.Notes on the Troubleshooting and Repair of Microwave Ovens
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If the problem was the result of a spill into the touchpad. it sounds like the controller is either not sensing the start command or refusing to cooperate for some reason . and then dry it out thoroughly. This is a long shot but might work. If you can locate the matrix connections for this button. Next confirm if possible that the START touch pad button is not itself faulty. Sharp Carousel IIs have a 'Minute Plus' button which will cook for one minute on HIGH. Fuse blows when microwave shuts off (during or at end of cook cycle). replacement will probably be needed. the timer would start counting. if you have nothing to lose. For example. Oven heats but shuts off randomly. and power setting. Testing the relay or triac control signal will likely show that it is not there. see the section: Erratic behavior. and would dump it otherwise. Oven heats on high setting regardless of power setting. The START button does.htm
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.

It may be in either connection to the HV transformer or elsewhere. Test for continuity.not present on most domestic ovens . Open HV capacitor . A shorted HV capacitor would likely immediately blow the fuse.This failure may also be due to loose. If this is blown. there will be no heating but no other symptoms.
Timer and light work but no heat. A number of failures can result in the fuse NOT blowing but still no heat: Bad connections .org/sam/micfaq. See the section: Testing and repairing the wiring and connections. Open winding in HV transformer. or deteriorated press (Fast-on) lugs for the filament connections and not an actual magnetron problem.repairfaq. oven light. high voltage fuses are somewhat rare on domestic ovens. Short in certain portions of the HV wiring. Defective HV relay.htm
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. a defective coil. A common location is at the crimp connections to the magnetron filament as they are high current and can overheat and result in no or intermittent contact. Open HV diode . A shorted HV diode. See the section: Testing the magnetron. and controller. (However.near zero ohms.see the section: Testing the high voltage diode.might blow. the HV fuse . This may have dirty or burnt contacts. A few models use a relay in the actual high voltage circuitry (rather than the primary) to regulate cooking power. or certain parts of the HV wiring would probably result in a loud hum from the HV transformer but will likely not blow the main fuse. See the section: Testing thermal protectors and thermal fuses. It should read as a dead short . It should read as a dead short . There are other variations depending on whether the cooling fan. burnt. Test for continuity.) Depending on design. or bad connections Shorted HV diode .Notes on the Troubleshooting and Repair of Microwave Ovens
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fuse has already blown there will simply be no heating action once the cook cycle is started.usually located on magnetron case. However. a number of other component failures could result in no heat as well including a defective relay or triac.near zero ohms. Open magnetron filament .see the section: Testing the high voltage diode. interlock switch(s). and so forth are located down stream of the fuse. Short or other fault in the magnetron .see the section: Testing the magnetron. or turntable rotation
This means the controller thinks the oven is working but the microwave generator AND motors
http://www. Open thermal protector . Some models may have a separate high voltage fuse. magnetron. See the section: See the section: Testing the magnetron. See the section: Testing the high voltage transformer.these may be almost anywhere in the microwave generator or the primary circuit of the HV transformer. cooling fan.see the section: Testing the high voltage capacitor.some ovens have one of these in the primary circuit. Open thermal fuse .

for example. This may be erratic. and the plastic 'fingers' which operate the interlock switches as well. that switch will not turn off before the other switches turn on shorting the power line. if the sequence is not correct. if its contacts are welded closed. the power line will be shorted blowing the fuse. one of the interlock switches is actually across the power line. Look for broken or dislocated parts. dislocated parts in the interlock mechanism. The rational for this basic design . Note that these symptoms are subtly different than just having no heat and eliminates the actual components of the microwave generator from suspicion in most cases. Similarly. This will prevent the oven from being used until it is inspected and repaired. (From: Bonita Lee Geniac (bgen@wdl.Notes on the Troubleshooting and Repair of Microwave Ovens
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aren't being powered. its mounting.is that a defect in the interlock switches or door alignment. There is also a slight possibility that the relay or triac on the control board is not closing but those usually do not result in these particular symptoms. which might result in dangerous microwave radiation leakage. There may be a burnt odor associated with this behavior:
http://www.
Loud hum and/or burning smell when attempting to cook
A loud abnormal hum is an indication of a short somewhere.some form of which is used in virtually all microwave ovens . occurring only 1 out of 10 times. Most of the microswitches used in recent production microwaves are very poor quality and the silicone lubrication used by some of the manufacturers migrates into the switch contact area and makes the switch fail even faster than it should. then such damage is quite likely. On some ovens. On others. If the oven was dropped. the power line will be shorted when the other switches close. As noted. there could be much more extensive internal damage as well. See the section: Testing and replacing of interlock switches. The sound may originate from the HV transformer vibrating and/or from within the magnetron depending on cause.org/sam/micfaq. it can only happen when interrupting the cook cycle by opening the door or when initiating the cook cycle from the front panel (if the switches are in the wrong state).htm
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. 99% of the time the lower door switch is bad or else the door is not closing fully and the latch hooks are not depressing the upper and lower switches. Marginal door alignment.) When the timer counts down but nothing else works. if the oven was dropped.repairfaq.
Fuse blows when closing or opening door
This means that the main fuse in the microwave (or less commonly. The cause is almost certainly related to either the door interlock switches or the door itself. and other indications of problems with the door and interlock mechanism Of course. If the switches are activated in the wrong sequence due to a misaligned door.net). or a defective interlock switch may result in either consistent or erratic behavior of this type. warpage. broken 'fingers' which operate the switches. will produce a hard permanent failure. this can happen at any time regardless of the control panel settings or whether the oven is in the cook cycle or not. Inspect the door. the fuse or circuit breaker for the power outlet) pops when the microwave oven door is closed or opened. Again.

) (Portions from: Tony (tonyb@ramhb.org/sam/micfaq. You have either Shorted HV capacitor. The other items listed below would likely blow the main fuse but possibly not always.Notes on the Troubleshooting and Repair of Microwave Ovens
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Shorted HV diode .nz). Shorted clamp diode across the HV Cap terminals (if one is present. Restart oven.) 4. If it still hums you probably have a faulty HV Transformer. Shorted HV Diode. Shorted magnetron (filament to anode) or other internal fault in the magnetron . or wiring on the load side of the HV capacitor. magnetron. Discharge the HV capacitor again. (The oven will run 100% without this protection for the
http://www. Arcing within the Magnetron case (visible through ventilation holes in the bottom section) is usually an indication of a bad magnetron. 5. go to step 4.see the section: Testing the magnetron. The following procedure will quickly identify the most likely component if the problem is not food/spills/carbon related: (Usually a loud hum that doesn't result in a blown main fuse is caused by a short in the HV diode. Short resulting from burnt on food (usually) in or around the waveguide.200 to 1. Note that a short on the load side of the HV capacitor will likely result in the actual wattage drawn from the power line being much lower than under normal conditions. Replace or get a new oven. (Not uncommon. Remove one end of the lead from the HV capacitor to the transformer. see the section: Arcing in or above oven chamber. Discharge HV capacitor! (If there is a short it is doubtful if it has any charge but never hurts to be safe). Hum still there? If so. 6.see the section: Testing the high voltage transformer. Hum Gone? If so. about 30% of microwave ovens use these).co. 2.see the section: Testing the high voltage diode. reconnect wire and disconnect the 2 wires to the magnetron. Hum gone? If so. Other short resulting from frayed insulation or wires touching in the microwave generator. A reading on an AC line wattmeter of 300 W compared to the normal 1.500 W would be reasonable.repairfaq. 3. Shorted HV transformer . Although there will be a high current flowing in the HV transformer secondary through the HV capacitor (which is what causes the hum or buz). magnetron is shorted.) 1. Start the oven. it is the HV circuitry. If the odor is coming from the oven chamber.htm
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. the real power consumed will be reduced since the current and voltage will be out of phase (due to the series capacitor) and the power factor will be low. go to step 6.

Usually.) Some older Panasonic ovens have a HV reed switch and these can also short but these are rare now because of the age. If your oven uses a stirrer above the oven chamber (no turntable). arcing. The result will be an uneven distribution of microwave energy and localized heating.Notes on the Troubleshooting and Repair of Microwave Ovens
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HV capacitor but it should be replaced if possible. jot down the locations of each of the screws (they are not always all alike) and stay away from everything but the waveguide cover itself (especially the high voltage components!). However. and possibly melting plastic or metal. particularly around and inside/above the waveguide cover. However. First.
http://www. the only way such damage could occur as part of the oven (not added knives or forks!) would be through physical abuse. The material can be obtained from places like MCM Electronics which you then cut to size with a pair of scissors or a paper cutter. usually mica. Clean the waveguide cover and clean inside the waveguide as well.org/sam/micfaq. it may be stuck.repairfaq. above.htm
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. There may be little plastic pins or snaps which tend to get gummed up with burnt food and may be difficult to pry off from inside the oven. inside. Then run the oven (with the waveguide cover removed. you need to clean inside the waveguide above the inside top of the oven as well. Trim to fit with a pair of heavy duty scissors. Any food that gets trapped here will eventually burn and carbonize resulting in a focal point for further arcing. and whatever of the cover material is remaining. if necessary) to verify that there are no other problems (there probably are none). burnt food and carbon often make this difficult so that some disassembly will be required. Sometimes. for example). If the waveguide cover is broken or damaged seriously. The oven will work fine without it but replacement will prevent contamination of the waveguide with food vapors or splatters which can lead to more expensive damage. Any sharp metal edges may also result in arcing or sparking. That waveguide cover is not essential to the operation of the oven but it does prevent food from entering the waveguide and getting trapped there. often just indicates that a thorough cleaning of the oven chamber is needed. If you do need to remove the metal cover. All traces of carbon and burnt on food must be removed. Take extra care to cover all food (which you should do anyhow) until the waveguide cover is replaced. completely clean below. Flashing and sparking may also result from the stirrer/fan blades contacting the metal surrounding it due to the motor/bearings becoming loose or dislodged. you need to remove the outside metal cover in order to remove the waveguide cover. metal snips. or a paper cutter. not a metal.
Arcing in or above oven chamber
There is often a simple cause: Arcing in the oven chamber with a normal load (a cup of water. See the sections: "SAFETY" and "Getting inside a microwave oven". the waveguide cover is designed to be removable without taking the (cabinet) cover off of the oven. a sheet of replacement material is available from places like MCM Electronics. In particular.
More on the waveguide cover and cleaning
That cover is made of an insulator transparent to microwaves.

However. The types of fuses used in microwave ovens are subjected to a heavy load and you may find that all that is needed is to replace the fuse with one with equivalent ratings.which you would think should blow the fuse probably will not do so because current will be limited by the impedance of the HV capacitor (assuming it is not shorted as well). and most interesting possibility. The following can cause the fuse to blow (in approximate order of likelihood): Defective interlock switches or misaligned door. If one side is defective. the main fuse will blow due to transformer core saturation since the triac will act as a rectifier and transformers really do not like DC. Old age or power surge.shorted or open. Defective HV transformer (shorted windings. There could be an intermittent problem as well which will only show up at some random time in the future. there will likely be a loud hum from the HV transformer as it strains under the excess load. See the section: Testing the high voltage capacitor. (but check for shorts first). A poorly timed power surge (as opposed to the well timed variety) could also weaken the fuse element resulting in eventual failure. Some ovens use a triac rather than a relay to control the main power to the high voltage transformer.see note below). Shorted HV capacitor. Shorted HV diode (see note below). Defective triac (shorted or partially shorted). Shorted wiring due to vibration or poor manufacturing quality. Fuses also die of old age. Recall that a triac is in effect a pair of SCRs in parallel in opposite directions. Such a sound in conjunction with no heat is a likely symptom of a shorted magnetron or HV diode. Alternatively. See the section: Testing the magnetron. or doesn't turn on or turn off reliably. See the section: Testing and replacing the triac. One type of failure of a triac is for it to be totally shorted causing the oven to come on whenever the door is closed. If your oven has a separate high voltage fuse somewhat rare in domestic ovens .
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Fuse blows when initiating cook cycle
The fuse may only blow when actually attempting to cook but depending on design. See the section: Testing the high voltage diode.org/sam/micfaq. Shorted magnetron (filament to anode . is that one half of the triac is bad . A third. Fuses sometimes blow for no apparent reason.it may certainly blow due to a fault in any of the HV components. See the section: Testing the high voltage transformer. At least one of the interlock switches is across the power line and will blow the fuse if not activated in the correct sequence. See the section: Testing and repairing the wiring and connections. See the sections: "Fuse blows when closing or opening door" and "Testing and replacing of interlock switches". the gate may be defective preventing the triac from ever turning on. Another possible cause of a blown fuse is a partially bad triac. Replace with exactly the same type and rating.repairfaq. triacs and/or door switches may always be live and may result in a blown fuse at any time when plugged in or when the door is opened or closed. The fuses used in microwave ovens are usually ceramic 1-1/4" x 1/4" 15 or 20 A 250 V fast blow type.htm
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. Note that a shorted magnetron or shorted HV diode .

00 from GE) with a new NTE56014 ($13. This is a fairly common problem in GE and Hotpoint models that use this board. Finally. a common cause that might not be obvious is that the triac used to switch power to the high voltage transformer is faulty.repairfaq.
http://www. The only other alternative is to replace the board. and I've seen it baffle many a repair shop.) The microwave oven in my General Electric JHP65G002AD cooking center blew its 15 AMP fuse each time the timing cycle expired. I replaced the triac with a $3 15 amp off-theshelf triac and it has been working for several years since. (From: Les Bartel lbartel@veribest.Notes on the Troubleshooting and Repair of Microwave Ovens
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See the chapter: "Testing and Replacement of Components" for more information on this and similar problems. and test the oven for proper operation. You can usually confirm the problem by setting the oven to a lower power level. Exactly how a bad relay could result in these symptoms unless it was actually arcing and shorting is unclear. The triac is probably located beneath a red plastic guard on the power control board. This is an indication of a weakened triac. (From: John Gallawa (microtech@gallawa.org/sam/micfaq. The following description applies directly to some GE and Hotpoint models." and heat a cup of water. install the outer cover. Its designation is usually Q1. Replacing the triac GE Part number WB27X5085 ($65. Depending on model.net). there is anecdotal evidence to suggest that inspecting the relay contacts and cleaning them if necessary may cure it in some cases. or SK 10265. core saturation. However. Drive to the triac could also be marginal but the bad triac is more likely.htm
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. You will probably hear a 'thump!' each time the magnetron cycles on.com). However.00.
Fuse blows when microwave shuts off (during or at end of cook cycle)
This could be due to a number of faults including shorting wires or defective relay.) I had the exact same symptoms on my GE microwave. say "medium. The cost used to be pretty reasonable.com).) I have seen exactly this problem. Modify it accordingly for your oven. Replace the triac (Q1) with either of the following: ECG 56010. the triac may be located on the control board or mounted directly on the chassis.00) solved the problem. See the chapter: "Testing and Replacement of Components" for more information on triac testing though replacement is probably the only sure test. and excessive current which blows the fuse. (From: John Montalbano (jrmont@iquest. It is likely that the triac on the 'Power Control Board' is breaking down. but now it's gotten expensive . What is probably happening is that only one half of the triac (recall that a triac is controlled for both polarities of the line voltage/current) is turning off completely resulting in DC to the HV transformer. replace the line fuse.probably about $80.

can you tell if it is actually heating continuously or rather it thinks you want LOW? Many microwave ovens make a clicking sound as they use a relay to switch microwave power on and off . This will probably show up with ohmmeter tests (with the oven unplugged!) but not always. Testing on HIGH will eliminate this possibility. 50% power should result in approximately equal on and off times. a failure of the controller or sensor (if you have one) could result in short cycling. Power output is quite sensitive to the AC input . Thus. are you sure the problem is real? Perhaps you are just a little less patient than you used to be. Perform a water heating test or try to pop a bag of popcorn using you usual time setting.check it. However. However. other settings are in between. There should not be any cycling on HIGH . Alternatively. can weaken with age and use. It is unlikely that any other electronic components could change value in such a way as to significantly affect power output. or controller. broken or lose belt if direct driven. However. you may see an unexplained variation in cooking times. its failure to rotate can result in hot and cold spots. for low. Mechanical problems are also possible. Make sure the magnetron is powered continuously and it is not cycling. The paddle is often accessible by unclipping a plastic cover above the oven cavity.the water heating test. the line voltage may be low. Replacements should be readily available. Magnetrons.check if you can hear this.org/sam/micfaq. lights on the same circuit or the oven light may dim slightly when the magnetron kicks in. it will be more difficult to diagnose as schematics for the controller are usually not readily available. First. See the section: Testing the oven .Notes on the Troubleshooting and Repair of Microwave Ovens
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Oven heats on high setting regardless of power setting
Power levels in a microwave oven are controlled by cycling the microwave generator on and off with a variable duty cycle .there is no regulation. it is not really possible to inspect for correct operation with the cover removed. When the oven always seems to be stuck at high power.the microwave power should stay on continuously while it is cooking. You can often tell by listening for the relay clicks and/or by observing the oven light/other lights dimming as the magnetron kicks in. If the problem is is the controller. you can put a microwave power indicator (NE2 neon light bulb with its leads twisted together) in the oven (with a cup of water for a load) and observe it through the window. it is likely to be due to one of two possible causes . it runs continuously. Check for bearing failure. etc. Where a spinning paddle wheel is used to 'stir' the microwave energy (often where there is no turntable). For 'HIGH'. If you run the oven on HIGH. If you are subject to brownouts or are running on your own generator. there may be a problem with the controller or you may unknowingly be in a low power mode . binding. Note that some are rotated by air flow from the cooling fan and require that cover to be in place to rotate. You should see a periodic
http://www. The relay or triac may have failed in the on state.kind of like slow pulse width modulation. If it is cycling.a faulty relay or Triac.repairfaq.
Oven heats but power seems low or erratic
Some considerations are how old the oven is and did the problem happen suddenly or did it just gradually weaken over the years.htm
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. it may run 10% on and 90% off. like other vacuum tubes. it could be something simple like a bad connection or dirty connector. Therefore. A 10% drop in line voltage is likely to reduce microwave power output by more than 20%. An oven that sees daily use may indeed weaken over the course of several years.

HV Diode. Inspect and clean and tighten (if necessary) all connections in the microwave generator including the magnetron filament. Be sure to unplug the unit first and discharge the HV capacitor before touching anything! The thermal protector may be intermittent.repairfaq. a bad motor. bad cooling fan (or just built up dust and grime block ventilation grilles). everything would appear normal but there would be no heating. There could be intermittent connections to the magnetron filament. or a controller problem. Should it gets stuck. but the oven shuts off after varying amounts of time. there could be a broken or weak belt.
http://www. It could be bad connections in the controller or elasewhere. HV transformer.htm
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. See the section: Testing thermal protectors and thermal fuses. But. or bad connections. some models may sense this and shut down/restart.
Oven makes (possibly erratic) buzzing noise when heating
Assuming operation is normal otherwise. other mechanical problems. Something may have loosened up with age and use. fan blades hitting something. Extremely high power line voltage may also result in overheating on a poorly designed or oven where the components are marginal. There may be something stuck under the turntable or above the waveguide cover interfering with the stirrer.org/sam/micfaq. thermal protector. Determine if the magnetron cooling fan is operating by listening for its sound or looking through the ventilation opening in the back of the oven.Notes on the Troubleshooting and Repair of Microwave Ovens
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variation in intensity as the paddles do their job. Make sure the stirrer fan is turning normally. if even for a few seconds. or some sheet metal or the high voltage power transformer laminations vibrating.. This could be a faulty magnetron. the filament opened). or elsewhere. then the magnetron could be faulty but check for the obvious cooling problems first: blocked or dirty ventilation grill. you would not be able to resume cooking until it cooled and the thermal protector reset. I would not suspect the magnetron or thermal problem as no cool down time is required. faulty controller. there would likely be a loud hum associated with the periods where there was no heat. If it is not possible to resume cooking for a few minutes indicating that something needs time to cool off. If the magnetron was overheating. this is most likely either a fan or other motor vibrating on its mounts. Test by clipping a light bulb across it or monitoring with a multimeter on AC voltage. Jiggle the door to see if this will cause it to shut off. HV capacitor. and thermal protector. If it is not. gummed up or lack of lubrication. a marginal door interlock switch. If resetting it allows cooking to resume immediately. If the magnetron were shorting. some other intermittent component. or bad connections.
Oven heats but shuts off randomly
Everything operates normally. If it just stopped working (i.e.not just a weak oven. these would likely show up as erratic operation .no heat at all sometimes . bad thermal protector.

Loose or broken belt. The solution may be as simple as tightening a screw or weging a shim between two pieces of vibrating sheet metal. 3. If open. Turntable.
Oven light does not work
If the oven light no longer works.g. These are typically not your usual vanilla flavored appliance bulbs either.it may be behind a non-removable grille requiring the removal of the cover. Disconnect one wire and check for continuity with an ohmmeter. Light bulbs may be typically located in any of 3 places: 1. Clean and lubrication as needed. no damage is likely to result. Confirm that belt is properly installed.Notes on the Troubleshooting and Repair of Microwave Ovens
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If the noise is caused be simple vibrations. Check for free rotation of the affected part(s). Oven chamber .repairfaq. Clean and/or replace if needed. It should return to its relaxed length instantly.always present.
Fans or turntables that do not work
There are up to 4 motors in a microwave oven: Magnetron cooling fan . This is the easiest.. Bad motor.the bulb may be in a recessed compartment accessible by removing a screw or two on the back of the oven. believe it or not. 2.stretch it by about 25%. not always so with microwave ovens. Rear . Test to determine if it is worn and flabby . a burned out light bulb is likely. at least identifying the cause is probably a good idea. if the main cooling fan is on its way out and it stops or gets stuck. the most likely causes are: Gummed up lubrication/dry bearings. parts will overheat quite quickly at which point the oven will shut down (hopefully) and there could be damage to the magnetron or other components. Unfortunately. turntable improperly installed).
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. When any of these do not operate properly. Bad connections are also possible but not that likely.org/sam/micfaq. Mechanical timer (on inexpensive non-touchpanel or older units). You would think that something like replacing a light bulb would be trivial and self evident. However. Also confirm that there are no other mechanical problems (e. Therefore. Convection air circulation (combo units only). winding is bad but check for break at terminal which you can resolder. Inside .it may be behind a mesh grill requiring a screw or snap to be removed.

replaced. or adjusted for any reason. I would NOT recommend making the repair in any manner that compromises the shielding properties of the door. For example. :)
What to do if the door handle breaks off
Usually this happens at the places where the handle is screwed to the door. Replacement door handles and/or entire doors may be available from the manufacturer of the oven.htm
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. or even superglue (though it seems not all brands are equally effective). replacing the screws with similar sized screws that gripped better or using filler to reconstruct or strengthen the threaded holes would be acceptable. Pry out the inner door trim with a small screwdriver on the latch side of the door.. not model cement).
http://www. WARNING: A microwave leakage test must be performed any time a door is removed. I would recommend staying with repairs that can be made totally externally unless there is no possibility of a change to the integrity of the door. Where a fan only runs when the oven is hot as in a microwave/convection oven. Plastic is generally tough to glue where a strong bond is needed and where the joint is subject to abuse. This is a "cool-down" function designed to allow the heat to equalize or possibly added by the company's legal department to reduce the number of lawsuits due to stupidity. (From: John Gallawa (microtech@gallawa. 2. Consider providing some reinforcements around the joint (i. Duco cement. Make sure the surfaces to be glued are perfectly clean (remove any residual library paste if you tried that!) and provide a means of clamping the pieces until the bond sets up (adhesive tape and/or rubber bands may be all you need). Locate the thermostat and jumper across its terminals with power off.a turntable and/or fan that runs after the cook cycle is completed may be normal for your oven.com).trace wiring and check continuity (unplugged. capacitor discharge) to motor terminals.e. 3. Bad connections .) Here are the door disassembly instructions from the Amana service manual. plastic cement (the kind that fuses the plastic. Plug the oven in and see if the fan now runs all the time or at least when the appropriate mode(s) are entered. (I have visions of someone using 1/2" stove bolts through the door and handle which would definitely be a bad idea). Remove six screws and release 4 spring fingers that secure the choke to the outer panel. Note that the opposite problem . PVC (pipe) cement.repairfaq. Therefore. Anything that penetrates the door seal is a potential hazard likely a very small one but it is not worth the risk. one or more of the following may work: semiflexible adhesive like windshield sealer. Replacements for a few Panasonic models are even stocked by MCM Electronics (and no doubt other places as well). plastic splints or sisters depending on your profession) for added durability. the thermostat or controller could also be at fault.Notes on the Troubleshooting and Repair of Microwave Ovens
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Bad thermostat. Many others are similar: 1. However. disassembled.org/sam/micfaq. Remove two screws securing the latch assembly and door handle to the outer panel (this may be all that's needed to replace the handle). depending on the type of plastic.

Even if it is not actually broken at this time. Is this safe to continue using or should I get it fixed? Will there be any radiation leakage?" So you were throwing roasts at the oven again. damage to the inner plastic is probably not a cause for concern as that is only there to keep the screen and inside of the door glass clean. future failure is possible. Use fine sandpaper to completely smooth out the metal and feather the edges of the paint in the immediate area. and damage to the interior paint. Use touch-up paint (with a small brush) or spray paint. However.htm
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. In this case. Also. sometimes splatters may find their way above the waveguide cover and cause problems above the roof of the oven chamber in the waveguide.more drastic action is called for: Assuming cleaning does not work on the carbon . the interior needs to be smooth. arcing.
Repairing damage to the oven interior
If spilled food . the glass panel or entire door should be replaced. Until you can obtain the paint.Notes on the Troubleshooting and Repair of Microwave Ovens
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Crack or other damage to door window
"My microwave oven has a crack in the glass of its door. The typical color is beige. Thus the warning not to use any metal utensils in a microwave. there could be shock hazard due to microwaves inducing current in the screen. poking something metallic through the screen would make is susceptible to microwave pickup as well. almond. there could be microwave leakage. clean up spills and food explosions as soon as possible. any break large enough to allow something to touch the metal screen is a hazard because during cooking. Not only will it be easier. If this happens in the vicinity of the mica waveguide cover.solid or liquid . the chance of future expensive problems will be minimized. but hot spots may develop and result in possible sparking. Not only will this be much more difficult to remove. To prevent arcing and sparking. However. Sharp edges and hard carbon in particular creates places where electric field gradients can become great enough to cause problems.repairfaq. Needless to say. the function of the glass is mostly cosmetic and a small crack should not be a problem. if the screen is inside the glass and now broken as well. In addition. This will probably damage the paint. Special microwave oven cavity paint is available but any common gloss enamel will work just as well (and costs about 1/10th as much).org/sam/micfaq. it may be damaged as well.just match it to your oven (if you care). or totally hardened impossible to remove carbon deposits . there is no danger.paint blisters and peels. it will tend to harden and carbonize.is not cleaned up soon after the oven is used. or some other form of off-white . huh? :-) If the metal screen/mesh is behind and separate from the glass. the oven will work fine but since the chamber is made of sheet
http://www.even after repeated attempts. And. Therefore. Once damage occurs . carefully scrape it off with a blunt knife or other suitable tool.

as you lower the resistance of the pot you should see the temperature readout climb. As a test. The heating element will be either a Calrod type (GE trade name?) which is a steel tube
http://www. So. waveguide cover mica sheets. a replacement will be required to assure continued safety with respect to minimizing microwave emissions.Notes on the Troubleshooting and Repair of Microwave Ovens
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steel. Possible choices include plastic or fiberglass laminate but not all materials will allow microwaves to pass without some heating .org/sam/micfaq. Remove and test with an ohmmeter. Also see the section: Sensor problems. During preheat. If the convection preheat cycle never completes and the oven is cool when opened. A convection oven which shuts down after a couple of minutes during the preheat cycle with the temperature display (if any) stuck at LOW (even though the oven is hot when opened) may have a bad thermistor temperature sensor. then the problem must be the heating element. Mica is also non-flammable which is may not be the case with other materials. The termister will usually be accessible after removing the oven cover. however. it must also not have any metal coating (don't use a piece of one of those 'browning disks' :-). parts like doors will need to be obtained direct from the manufacturer. For most ovens. An infinite reading means it is bad. do paint it. It is a two terminal device that may look like a tiny resistor or diode and may be mounted on a metal header fastened with a couple of screws. Of course. jumper a 50 K ohm potentiometer in place of the thermistor. If the interior of the door is damaged seriously such that either it will not longer seal around the edge properly or that the mesh screening is breeched. then either the heating element is bad (test with an ohmmeter) or the relay controlling the heating element or the controller itself is bad. If the material doesn't heat up. Microwave oven cavity paint. Leaving it larger than necessary is fine as well. Replacement thermistors are available from the oven manufacturer . Alternatives to mica which can stand the elevated temperatures in a microwave oven may also be acceptable. it should be fine.such that it no longer will prevent splatters from entering the waveguide. The overtemperature protection sensor (rather than the normal temperature sensor) is shutting the oven down.
Microwave/convection oven problems
In addition to the microwave components.check it out. If the circulating fan runs off of the same relay and it is operating.about $20. Use a suitable bit in a hand drill to make holes in the mica for the mounting screws or plastic snaps.repairfaq. It will be located centrally just above the oven ceiling duct or elsewhere in the convection air flow. these ovens also include an air circulating fan and an electric heating element as well as a temperature sensing themister. The oven will then indicate READY when the simulated temperature exceeds the setpoint. Heat a cup of water and the candidate material on high for a couple of minutes. Any of these can fail. cut to fit. and even some replacement doors are available from the parts suppliers listed at the end of this document. obtain replacement material. If the waveguide cover is damaged seriously . Cheaper alternatives may be possible but you would need to know the exact specifications and it is probably impossible to obtain this information. rust will set in eventually.htm
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. below.

Substitute a fixed or variable resistor and see if you can get the oven to shut off (or stay on) as a function of resistance. this is generally not convenient.
Sensor problems
Fancier microwave or microwave/convection ovens include various probes that can be used to shut off the oven when the food is supposedly done or maintain it at a preset temperature. ERROR. Temperature probes may use a thermistor similar to one that controls the convection portion of a microwave/convection oven. Inspect the belt. though it is worth trying an appliance parts distributor or a place like MCM electronics first. The former is probably only available from the oven manufacture. However. If it is loose. It may only be active in certain modes. See the section: Microwave/convection oven problems for a discussion of thermistors. Steam/humidity probes may also behave similarly. A problem with a sensor. controller. The circulating fan is probably driven by a belt. The problem is the temperature sensor thermostat located on the top rear of the oven. the sensor and the probe cable are the primary suspects.org/sam/micfaq. CAUTION: Don't forget to put a cup of water in as a load if you are testing microwave operation. or does not return to its normal length instantly after being stretched by 25% replace it. sensor for the correct oven temperature.Notes on the Troubleshooting and Repair of Microwave Ovens
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enclosing a Nichrome wire coil embedded in ceramic filler or a coiled Nichrome element strung between ceramic insulators. It may be possible to find a replacement Nichrome coil and form it to fit. which may break or deteriorate. It may be high (hundreds of K ohms) but probably should not be open.repairfaq. Check for bad connections where the probe plugs in as well as broken wires inside the cable particularly near the ends where it gets flexed. There should be some resistance when measuring between the signal conductors of the probe cable. A very low value (a few ohms or less) might indicate a short in the cable or sensor." When problems develop with these automatic features. Replacing this open sensor will correct the problem. Make sure the wire gauge and length are identical.htm
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. The best test of the probe unit is to substitute a known good one. etc. Testing to determine if the controller is responding to the input from the sensor can be done in a similar manner except that access must be from inside the electronics bay while the oven is running (the probe normally plugs in inside the oven chamber).
http://www. If you have never tried the probe before. cracked. it is possible that the electronic circuitry could also be affected by a damaged or defective probe unit. or something similar: (From: Wilton Itamoto (witam40231@aol. may result in incorrect operation (never getting past 'preheat' or not terminating a cook cycle) or in a display of 'EEEE'.com). This is the convection temp.) "The 'FFFF' display is a common problem in older Panasonic convection ovens. or wiring. Check the fan motor and fan itself for adequate lubrication. Of course. check your users manual. Check the fan blades for corrosion and damage. 'FFFF'.

fat chance of that!) You can skip the heavy math below and jump right to the final result if you like.) which is 8 x 29. Or.) 1 minute equals 60 s (but you know this!). Therefore. if your prefer Fahrenheit: 141 °F.
Testing the oven . Scale the expected temperature rise by the ratio of the microwave (not AC line) power of your oven compared to a 1 kW unit. a very simple test is to place a measured cup of water in the microwave from the tap and measure its temperature before and after heating for exactly 1 minute on HIGH. for those who are interested: 1 Calorie (C) will raise the temperature of 1 gram (g) of liquid water exactly 1 degree Centigrade (DegC) or 9/5 degree Fahrenheit (DegF). from a Litton microwave handbook:
http://www. a 1 kW microwave oven will raise the temperature of 1 cup of water by:
T(rise) = (60 s * 1000 J/s * 0. However.184 Joules (J) or 1 J = 0. Faulty circuitry in the controller is also possible.239 C. 1 Watt (W) of power is 1 J/s or 1 kW is 1000 J/s.6 g) = 60.htm
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. Therefore. look for corrosion or other deterioration of the socket in the oven chamber as well as bad connections. = 236. If the probe checks out or substituting a known good one makes no difference in behavior. To account for estimated losses due to conduction. in one minute. and imperfect power transfer.6 °C. (For Avoirdupois ounces.oz.
Or. than a bad probe unit is likely. 1 Calorie is equal to 4.239C/J * (g * DegC)/C)/(236. 1 cup is 8 fluid ounces (fl.oz.repairfaq.
Back to Microwave Oven Repair FAQ Table of Contents. A couple of minutes with a cup of water and a thermometer will conclusively determine if your microwave oven is weak or you are just less patient (or the manufacturer of your frozen dinners has increased their weight sure.6 g. convection.the water heating test
The precise number of degrees a known quantity of water increases in temperature for a known time and power level is a very accurate test of the actual useful microwave power. I suggest using temperature rises of 57 DegC and 135 DegF.
Testing and Replacement of Components
Please see Typical Microwave Oven Electronics Bay for parts identification.org/sam/micfaq.35 g.57 g/fl.Notes on the Troubleshooting and Repair of Microwave Ovens
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If the resistor test determines that the controller is responding. use 28.

(You may need to disconnect one side of the transformer primary since its resistance is a fraction of an ohm.
Testing and replacing of interlock switches
With the oven unplugged. for low power tests. the main fuse is the place to start: UNPLUG THE OVEN and locate and remove the main fuse. or high voltage wiring.org/sam/micfaq.) The intermediate power levels can be tested as well. suspect problems with the interlock switches. The heating effect of a microwave oven is nearly linear.htm
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. Oven power = temperature rise in DegC multiplied by 70.repairfaq. Test it with an ohmmeter . However. current rating). then the test is invalid use colder water or a shorter time.) Replace with switches having a precisely identical fit and equal or better electrical specifications (terminal configuration. put an ohmmeter across the AC input just before the interlocks (but beyond the power relay or triac if it precedes these). Refer to the schematic pasted inside the cover. a cup of water should take nearly roughly twice as long to heat a specific number of degrees on 50% power or 3. It will usually be a 1" x 1-1/4" ABC ceramic type directly in-line with the Hot (black wire) of the power cord. (Note: if the water is boiling when it comes out .may indicate a problem. If it is good but the oven makes a loud humming sound when you attempt to cook. increasing the time to 2 minutes with 2 cups of water will result in more accurate measurements due to the long period pulse width power control use by microwave ovens which may have a cycle of up to 30 seconds.3 times as long on 30% power as on full power. Thus. If it is blown. There will be some losses due to convection but this should not be that significant for these short tests. Use a plastic container rather than a glass one to minimize the needed energy loss to raise its temperature by conduction from the hot water.Notes on the Troubleshooting and Repair of Microwave Ovens
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Heat one Liter (L) of water on HIGH for 1 minute. and poke your thermometer through it. For the ultimate in accuracy (as these things go). If it approaches zero while opening or closing the door. put the water in a styrofoam cup. (Due to conduction and convection losses as well as the time required to heat the filament of the magnetron for each on-cycle. suspect the magnetron or high voltage diode.
Testing the main fuse
Where the oven is dead or mostly dead. When removing the old switch make a note as to where each
http://www. the interlock switches and door alignment should be checked. Any significant discrepancy between your measurements and the specified microwave power levels say more than 10 % on HIGH . the accuracies of the intermediate power level measurements may be slightly lower). invert another styrofoam cup over it. Open and close the door slowly several times there should be no significant change in resistance and it should be more than a few ohms. See the section: Oven heats but power seems low or erratic.the reading should be zero ohms. high voltage capacitor.at 100 DegC or 212 DegF.

org/sam/micfaq..a switch may just need to be popped back into place. check the interlock mechanism first .not even if your meter is being consumed by 5 foot flames! First. Even professionals have been killed performing measurements of this type using proper equipment! Luckily.Notes on the Troubleshooting and Repair of Microwave Ovens
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wire goes. Get the proper equipment! One thing you can do relatively safely is to connect a Variac directly to the primary of the HV transformer. here are some guidelines to a long life: WARNING: ALWAYS pull the plug and discharge the HV capacitor BEFORE doing anything inside! Never be tempted to make any changes of any kind while the oven is on . I only include this section for those who really want to know the details. If it is very low or 0. However. Magnetron current . Note that a shorted as well as open magnetron also results in no current. Measure the voltage drop across this resistor. A scope can also be used if it has a proper 10:1 probe as long as you aren't tempted to turn up the Variac any higher! The scope waveform should be close to a sinusoid with its positive tips at 0 V. pull the plug and discharge the HV capacitor! High voltage . You may be temped to break out your Radio Shack DMM and start poking away inside a live microwave oven.g.htm
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. it bypasses all
http://www.repairfaq. current measurements can provide enough information to help make a diagnosis. or a proper microwave oven tester . SET EVERYTHING UP AND THEN STAND BACK and don't forget to DISCHARGE the HV capacitor after making the measurement: If it is around this range. (Another reason to stay calm after accidentally nuking that bagel for 5 minutes on HIGH!) So if there was some kind of "event" after which the microwave failed. no measurements of any kind on the oven while it is operating will be needed to identify and correct the problem. If the magnetron is shorted. where this is not the case. Sensitivity will be 10 V/A. Such reduced voltage tests won't identify problems that only occur at full voltage. for testing CRT HV) and no polarity switch! The ground cable doesn't have anywhere near the required insulation. the voltage on the filament terminals of the magnetron should read from -150 to -250 V with respect to the chassis. DO NOT be tempted to interchange the probe and ground wire if you are using a high voltage probe on a meter with a POSITIVE input (e. DON'T! This isn't like a CD player! Most of the time. resulting in breaker tripping at the electrical service panel whenever the microwave oven door was closed.don't depend on location as your replacement might just have a different arrangement. the magnetron is probably fine.DON'T even think about this unless you have a proper high voltage probe or meter. This will be -3 to -4 VDC across the 10 ohm resistor with respect to chassis ground.AND KNOW HOW TO USE IT SAFELY. With this set at a MAXIMUM of 10 percent. Normal anode current is around 300 to 400 mA for a typical oven. Check the embossed marking on the old switch . however.Place a 10 ohm 10 watt resistor in series with the HV diode cathode and ground. Make sure the new switch aligns correctly with the actuating mechanism and then check for correct electrical operation with an ohmmeter before applying power.
Making measurements inside microwave ovens
WARNING: In general. Even slamming the door really hard has been known to knock an interlock switch out of position. I DO NOT recommend making any sorts of measurements on the high voltage components of a live microwave oven. magnetron is bad or HV is not working. WARNING: The high voltage components inside a microwave oven are at a NEGATIVE potential with respect to the chassis.

with power disconnected.htm
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. Assuming the oven passes the above test for interlocks and door alignment. relay. it may also be possible to replace only the one that is found to be defective or make up a substitute HV cap/diode assembly from individual components if the combined unit is excessively expensive or no longer available. HV capacitor.) A properly conducting magnetron will load down the HV power supply. the most likely problems are in the microwave generator. See the section Safe discharging of the high voltage capacitor. With full conduction by the magnetron.ca).) is to connect the HV transformer primary directly to a line cord and plug. HV diode. etc. discharge the high voltage capacitor. magnetron. the HV capacitor charges up and then there is no more current through the HV diode (but there will be an initial transient). If the magnetron is nonconducting.100 V. open circuit (as when the oven is first turned on and the magnetron filament/cathode is not fully heated). Test the filament for continuity . it should be possible to test each component individually. (Again.it should be high in at least one direction. (The voltages vary with design and model. or magnetron are a dead short (as well as for an open magnetron filament). the resistance in at least one direction should be several M ohms.100 V. use the higher reading). However. depends on the circuit. The magnetron may short out when full voltage is applied.Notes on the Troubleshooting and Repair of Microwave Ovens
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current to ground. Tape the removed wire lugs to prevent shorts. or more. Don't overlook the wiring as no heat or erratic operation can result from simple bad connections! An alternative way of determining if the problem is in the control circuits (triac. capacitor is shorted.
http://www. If it is much too high (whether fuse blows or not).repairfaq. but the HV remains well above the 2. Where the capacitor and diode are combined into one unit.they can definitively identify parts that are bad but will not guarantee that they are good. the voltage remains high.) The HV at the magnetron filament is negative to ground. Use an ohmmeter to test the diode and capacitor. but I think this is a common configuration. measuring between the magnetron filament connectors (either one) or at another equivalent point. If the magnetron is open. Weak magnetrons conduct somewhat. These may be considered to fail/no conclusion tests .carleton. wiring) or microwave generator (HV transformer. The power supply will produce 3.800 and 2. the HV drops to between 1. (From: Michael Caplan (cy173@freenet. While connected in circuit. but the magnitude of the change is the key. An ohmmeter can be safely used to quickly determine if the capacitor. wiring.) I check the HV using my 30 kV HV probe with a DMM.500 to 4. Connections may open up when they heat up.000 volts DC. There could also be a wire shorting to the chassis. and case ground. Parts may test ok with no voltage applied but then fail once operated in-circuit.the resistance of a good filament is close to 0 (less than 1 ohm). (Try it in both directions.
Testing the high voltage components
WARNING: First. the triac (if used) may be defective. In some cases. Test the magnetron from the filament to chassis .org/sam/micfaq. HV diode.

If the problem is with the triac or its drive. This will determine proper behavior. An open HV diode will result in AC instead of DC across the magnetron with a peak negative value (the only one that matters) about 1/2 of what it should be. The result will likely be little or no detectable heat but no other symptoms. Power the oven via its line cord. The resistance measured across the leads of the HV diode should be greater than 10 M ohm in at least one direction when disconnected from the circuit. switch on the HV transformer. It is not likely for there to be anything in between as so much heat would result that the diode would not remain that way for long.500 W would be reasonable. See the section Safe discharging of the high voltage capacitor The HV diode can fail shorted (most likely) or open. Its forward voltage drop will therefore be too great (6 V or more) for a DMM to produce a definitive answer as to whether it actually works as a rectifier. Check the water's temperature. Put a cup of water into the oven cavity to act as a load. The HV diode can be tested with a DC power supply (even a wall adapter of at least 12 or 15 V output). the HV diode is composed of multiple silicon diodes in series to get the voltage rating. something in the controller or its wiring is shorted. However. Or. A reading on an AC line wattmeter of 300 W compared to the normal 1. A shorted HV diode will likely result in a loud hum from the HV transformer when a cook cycle is initiated. discharge the high voltage capacitor. note that the actual wattage drawn from the power line will probably be much lower than under normal conditions. and your multimeter. initiate a 1 minute cook cycle on HIGH and with the oven running. The following is the schematic of a simple HV diode tester:
http://www. It should go through the normal cycle (of course no heat) without blowing the fuse or any unusual sounds. switch off the outlet strip. with power disconnected. The main fuse will probably not blow. the oven will now heat normally.org/sam/micfaq.200 to 1. Initiate a cook cycle.Notes on the Troubleshooting and Repair of Microwave Ovens
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Plug the transformer cord into a switched outlet strip which includes a fuse or circuit breaker. When the cook cycle is near its end. series resistor (to limit current). Although there will be a high current flowing in the HV transformer secondary through the HV capacitor (likely causing a loud hum or buzz). it will likely dim significantly due to the heavy load before the fuse or breaker cuts out. However. if a lamp is plugged into the outlet strip at the same time. Now. If there is a problem in this case. the real power consumed will be reduced since the current and voltage will be out of phase (due to the series capacitor) and the power factor will be low.
Testing the high voltage diode
WARNING: First. If the transformer or other HV components are faulty. at least at low voltages.repairfaq. More complete information on testing and replacing the individual components is provided in the next few sections. the outlet strip fuse will blow or circuit breaker will trip.htm
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.

Replacing the HV diode
WARNING: First. the HV diode. and possibly even a defective magnetron or damaged waveguide. Or. the reverse voltage goes down somewhat. It may be a good idea to solder the lugs to the wires as well (though this may be overkill).org/sam/micfaq.Notes on the Troubleshooting and Repair of Microwave Ovens
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240 ohms. Here is why: Until the magnetron heats up and starts conducting in its forward direction.500 VAC.5 A.. and some power is
http://www. This can easily be 6 or 7 kV or more! Once the magnetron start conducting. but no other symptoms either. it may be necessary to eliminate the other components one by one. A PRV of around 8 kV is actually required even for a small oven. no heat. HV diodes rated at . Where the diode is part of the capacitor assembly.repairfaq. Discharge the high voltage capacitor.
HV diode ratings
Most replacement microwave oven diodes are rated 12 to 15 kV PRV at .which is likely. it is possible for failures elsewhere to have caused the diode to blow. with power disconnected. For example. Most HV diodes have press fit (Fast-On) or ring lugs so replacement is very straightforward. Putting the diode in backwards will result in positive instead of negative high voltage and. While there are some losses in the HV transformer. charring between the HV transformer windings or hole burned in the waveguide).g. If this is the case. These may only occur with full voltage so unless there is obvious physical damage (e.414 * (VRMS of the HV transformer). arcing between windings in the HV transformer. Although a shorted HV diode is usually an isolated event. Make sure you get the polarity correct if your replacement can be installed either way. what you have is a half wave rectifier/filter formed by the HV transformer secondary. Note: the lugs on your new HV diode may just be crimped onto the wire leads and not welded or soldered. take care not to stress them excessively which might result in bad connections now or in the future. 1 W + o-----------/\/\---------+------------o + | __|__ HV Good: 6 to 10 V 15 VDC _\_/_ diode Shorted: 0 to 2 V | Open or reversed: 15 V | . and the HV capacitor. If your DMM or VOM has a resistance scale operated off a battery of at least 6 V. discharge the high voltage capacitor. needless to say.o------------------------+------------o -
The voltage drop in the forward direction should be at least 6 V with a few mA of current but may be somewhat higher (8 V or more) with a few hundred mA. This will probably be much much cheaper than replacing the entire assembly. it may be possible to just replace the diode leaving the old one unconnected (at one end) as long as the original diode isn't tied to ground inside the case. Possible causes include a shorted HV cap. you may get a reading in one direction (but only one) without the need for an external power supply.800 W power line input and a HV transformer secondary of 2. the largest of these will have a nameplate rating of around 1.htm
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. assume for now that the diode is good if it is not shorted . The reverse voltage across the HV diode will be equal to: 2 * 1.5 A are adequate for most domestic microwave ovens. See the section Safe discharging of the high voltage capacitor.

An open HV capacitor will result in no heat but no other symptoms. and then check the HV diode for overheating. Just make sure the ratings of the capacitor and diode are correct (use a generic replacement microwave oven HV diode and a microwave HV capacitor with a uF rating within 10% or so of the old one and at least equal working voltage). if you measure 0. this test is probably not needed.org/sam/micfaq. without a shadow of a doubt. you don't need to use the exact combined part .600 W into the HV generator. To be doubly sure that your new HV diode is happy. Unplug the oven (while your spouse prepares the veggies). Even this does not prove that it will not short when full voltage is applied. 1. Unless you have the largest oven on earth. However. Where the capacitor assembly also includes the HV diode. Thus.repairfaq. Substitution is the only sure test beyond this. (The following assumes no internal rectifier or other circuitry except of a bleeder resistor. the capacitor is definitely shorted. the cost of a generic replacement diode is small (around $3) so replacing both at the same time is usually best. Replace in reverse order. discharge the high voltage capacitor.) The resistance measured across the terminals of the high voltage capacitor should be very high several M ohms for bleeder resistor. closer to 1/2 of the total current actually flows through the HV diode. If you have a capacitance meter.5 A is not enough. Make a diagram of the precise wiring as multiple connections are often made to the capacitor terminals. It might be warm but should not be too hot to touch. However. not all the power flows through the HV diode (as would be the case with a regular power supply. due to the design of the half wave doubler circuit.
http://www. However. The capacitor is usually mounted with a clamp which is easily loosened. with power disconnected. and light. discharge the high voltage capacitor.00 ohms across the terminals (and they are not bussed together on the case).
Replacing the high voltage capacitor
WARNING: First. just not shorted with no voltage across it. motors. Tighten the clamp securely but not so much as to distort the case.Notes on the Troubleshooting and Repair of Microwave Ovens
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used by the magnetron filament.64 A) would suggest that . Yes. controller. the capacitor is jammed into a location that requires moving some other components to extract it. A shorted HV capacitor will blow the fuse instantly. run the oven on full power (high) for 10 minutes with two quarts of water as a load (or a roast). perhaps. quickly DISCHARGE THE HV CAPACITOR. See the section Safe discharging of the high voltage capacitor. Sometimes.htm
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.500 or .600/2. Adjust procedures accordingly if your oven is different. this still leaves. check it for proper value (should be printed on the case). bad! A high resistance does not prove that the capacitor is actually functional. See the section Safe discharging of the high voltage capacitor.which may be very expensive or difficult to obtain. then the capacitor is positively. If it is less than 1 M ohms. even though calculations using Ohms law (I = P/V = 1. with power disconnected.
Testing the high voltage capacitor
WARNING: First. it is possible to just replace the capacitor if space permits leaving the old one unconnected (at one end).

500 W would be reasonable. See the section Safe discharging of the high voltage capacitor.
http://www. gently tap the magnetron to determine if there is an intermittent short.200 to 1. There is no totally definitive way to determine if a magnetron is good without actually powering it under operating conditions but the following tests will catch most problems: Magnetron filament. The ceramic insulators are translucent and should show a glow with a working filament. However.g. a microwave oven transformer but just the filament connections). A magnetron with other faults may result in a variety of symptoms including erratic or low output power or intermittent operation. It may be possible to determine if the magnetron filament is actually working by connecting just the filament connections to a low voltage high current supply on a Variac (e. until this happened. note that the actual wattage drawn from the power line will probably be much lower than under normal conditions. I consider these sorts of failures somewhat unlikely as the HV diode and capacitor do not generally fail half-way!
Testing the magnetron
WARNING: First. the real power consumed will be reduced since the current and voltage will be out of phase (due to the series capacitor) and the power factor will be low. with power disconnected. A reading on an AC line wattmeter of 300 W compared to the normal 1.repairfaq. A magnetron with an open filament will result in no heat but no other symptoms. The resistance should be infinite from the filament connections to the case and a fraction of an ohm between the filament terminals with the wiring disconnected from the magnetron. such problems may only show up once the filament heats up and parts expand.htm
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. While measuring resistance from filament chassis. The one at the antenna may be visible if the magnetron is removed from the oven or with a dental mirror looking into the waveguide. A magnetron with with a short between the filament/cathode and anode will likely result in a loud hum from the HV transformer and/or magnetron when the cook cycle is initiated but the main fuse will probably not blow. The effect on performance in both cases would be to reduce the effective voltage across the magnetron and thus the output power. discharge the high voltage capacitor.. However. The bad connection may be internal (in which case the magnetron will need to be replaced) or external at the filament terminals (which may be repairable).org/sam/micfaq. See the section: Comprehensive list of magnetron failure modes.Notes on the Troubleshooting and Repair of Microwave Ovens
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What if the HV diode or capacitor are leaky?
An (electrically) leaky HV diode or cap would likely fail totally in short order since it would be dissipating a lot of power. However. the oven might continue to operate and not blow a fuse. WARNING: Make sure you ONLY have the filament connected! Evidence of arcing (visible blackening around ventilation holes in base or burnt odor) usually indicates a bad magnetron. Although there will be a high current flowing in the HV transformer secondary through the HV capacitor (likely causing a loud hum or buzz).

loud hum when entering cook cycle. Magnetron could be gassy (or up to air) and arcover internally once power is applied. Filament could be open . Symptoms: No heat. internal shorts and loose filament connectors are probably at the top of the list. Tap the tube while measuring to check for intermittents. Currently. Shorts. check directly at the magnetron terminals with both lugs pulled off. Overheating might result from a broken or cracked magnet (reduced magentic field) or other internal problems. however. (The shape doesn't matter as long as it fits tightly . If you had one.there are several diameters.check with ohmmeter. Anything less than infinity means the tube is bad though it could be charring due to arcing outside the vacuum in the box with the filament connections.
Comprehensive list of magnetron failure modes
(Portions from: John Gallawa (microtech@gallawa. Internal or external arcing resulting in physical damage. the damaged antenna cover can be pulled off and replaced from a magnetron that died of other causes .repairfaq. This fault isn't really likely. While there may be some output power. 1. If a problem elsewhere has been corrected. External arcing could be at the antenna or inside the filament box. (This part is only visible with the magnetron removed from the oven). Filament could be shorted to itself . (a) Internal plate-cathode/filament short or (b) Internal arcing.htm
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.check with ohmmeter. (Yeh.Notes on the Troubleshooting and Repair of Microwave Ovens
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Melting or other damage to the antenna cover ('bull-nose' or 'bullet') may be the result of arcing due to problems in the oven cavity or waveguide (perhaps operating with nothing in the oven) or a defective magnetron.tough to test since it is such a low resistance to start. it will test as a dead short to the case on your multimeter using DC and is normal. Therefore. There is no easy way to test for these possibilities other than substituting a known good magnetron. Internal arcing will not leave any visible evidence but the damage will result in the magnetron failing totally or running with reduced output.) Here is a list of typical magnetron failure modes. this wouldn't be an issue!) Tap the tube while measuring to check for intermittents. The percentage of each type of failure varies.try your local appliance repair shop. Compare with good magnetron. this won't be the case! :) Most common magnetron failure modes: Filament could be shorted to case . The filament could expand.) Your magnetron may still be good. and short once heated. Note: Since the antenna is attached directly to one of the vanes which is part of the anode assembly. Tap the tube while measuring to check for intermittents. loose filament connectors (Fast-Ons) are more likely than a broken filament.
http://www. possible blown HV fuse (but will not likely blow the main fuse). An internal plate-cathode short may only manifest itself under the stress of high voltage during operation. At 2. However. shift position.com). right. the thermal protector will shut down the oven prematurely.45 GHz.org/sam/micfaq.

possible blown HV fuse. (I suppose the transformer absorbs most of the current surge. Symptoms: No heat. loud buzz due to arcing when entering cook cycle. If the the magnetron terminal(s) have not been burned too severely. the vacuum envelope can rupture. possible blown HV fuse. the connection(s) can usually be repaired. its RF gasket or waveguide flange. with reference to the other symptoms below. Note: when discharging HV capacitor. loud hum once it occurs. Open filament. Usually occurs after a few minutes of normal operation. (b) RF interference. Filament breakdown. Low output. Symptoms: No heat. build up resistance and eventually loose contact. the high voltage fuse would probably blow. The slip-on connectors can loosen.) In fact. 2. But. rarely will a shorted magnetron cause the main line fuse to blow. there are almost no failures where the magnetron causes the line fuse to blow. Occurs when magnetron oscillates in one or more undesirable frequencies. See note about HV capacitor in (2) above.htm
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. We prefer cleaning up the terminal. 6. See comments about fuses in (1) above. See comments about fuses in (1) above. Symptoms: (a) Reduced or no cooking power. or its RF (feed-
http://www.repairfaq. Moding. Occurs as cathode emission decreases from long use.Notes on the Troubleshooting and Repair of Microwave Ovens
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In ovens equipped with fuses that monitor the high voltage system. then soldering the filament wires directly to the terminal. it may end up being charged to a much higher voltage than is normal. Be prepared for a larger spark if you use a screwdriver to discharge it! 3. Symptoms: Reduced cooking power. 4. some food products (with high water content) may cook normally. There will often also be visual symptoms at the magnetron: Signs of overheating. such as discoloration. such as some commercial Sharp models and most commercial and domestic Amana models. Symptoms: No heat. An intermittent filament (internal) is also possible (but not repairable). RF interference is possible but usually only occurs if there is actual structural damage to either the magnetron. whereas the result with other foods is very unsatisfactory. Symptoms: No heat or erratic heat. However. and evidence of carbon tracks or pits on magnetron terminals when the connectors are removed. 5. since there is no load. In the older glass-dome models. Loose filament connectors (these may be repairable). overheat.org/sam/micfaq. 7.

Where to obtain replacement magnetrons
Depending on the age of your oven the magnetron may still be under warranty. such a replacement may be more than half the cost of a similar new oven. Symptoms: Microwave leakage into electronics bay. quality may vary. erratic control panel behavior. Physical characteristics can change and cause magnetron to oscillate at frequencies slightly higher or lower than 2. Generic replacement magnetrons are available for the majority of microwave ovens. original magnetrons may also be available from parts suppliers like MCM Electronics . These will almost certainly be much less expensive than original parts.htm
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. a copy of the sales receipt or other proof of date of purchase may be required. However. the escaping RF energy is confined.repairfaq. Cracked magnet(s). Insulation breakdown of the internal leads or at magnetron insulators or antenna terminal. Check the original paperwork that came with the oven . Symptoms: Reduced or no cooking power. Going direct to the oven manufacturer will guarantee a compatible magnetron but is by far the most expensive option. who actually does this?!). and eventually builds up around the control panel circuitry causing unusual symptoms. In some cases (like Sears).45 GHz. Structural failure can cause leakage from magnetron housing.Notes on the Troubleshooting and Repair of Microwave Ovens
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through) capacitors. With the cover in place. no heat.at somewhat less rediculous prices. In some cases. burning smell from magnetron. magnetron overheating. one without the gold-plated trim :-). Essentially. occasional 'snapping' sound. The differences are mostly mechanical. If you have not sent in the warranty registration card (right. loud hum.com). Contact the manufacturer if specific instructions on how to file claims are not provided. 9. Same as (7a) above. there is only one type 'tube' (at least for any similar power range). the generic variety may actually be better than the original. They will be identified as 'original' or 'genuine' along with the manufacturer and their part number. Full coverage on the magnetron of several years is common. you may need to convince their service department that you are qualified to be poking around inside one of *their* appliances before they will consider selling one to you (too many lawyers).org/sam/micfaq. Both original and generic replacement magnetrons are available. 8. In some cases. 10. Off frequency. For a typical oven.either the users manual or a separate warranty document. Symptoms: Arcing.
Comments on replacement magnetron quality
(From John Gallawa (microtech@gallawa. RF leakage.)
http://www. See the section: Comments on replacement magnetron quality for some recommendations. It can be very frustrating because the symptoms disappear when the oven's outer cover is removed. 11.

doesn't matter. discharge the high voltage capacitor. When buying magnetrons from other than the manufacturer. These sales people are usually more knowledgeable about the magnetrons they sell. However. AMI.org/sam/micfaq. See the section Safe discharging of the high voltage capacitor. When you receive the replacement. The studs may be removable so that the same assembly can be used with or without them. :-)
Testing the high voltage transformer
WARNING: First. such as Toshiba and Hitachi. though: In many cases. loud hum. The shape of the antenna terminal . mags purchased from after-market suppliers may or may not be OEM parts (there are not that many manufacturers of magnetrons in the world). A shorted winding or short between a winding and the core/chassis in the HV transformer may result in a blown fuse. see the section: The magnets in dead magnetrons. with power disconnected.e. Of course. Global Micro-parts. I have found it best to go to a supplier who specializes in microwave oven parts (i. compare it with the original. the magnetron is fastened to the waveguide with 4 nuts on studs. See the section Safe discharging of the high voltage capacitor.up to half of the input power to the magnetron ends up as heat. depending on the after-market supplier.a metal mesh ring which seals the connection against microwave leakage. An open winding will likely result in no heat but no other symptoms. Magnetron replacement is generally straightforward but other assemblies like the cooling fan may need to be removed to gain access. I have seen the low-end tubes in many brand-new microwave ovens. waveguide seating surface. and the orientation of the filament connections and cooling fins are the same. The cooling fins are particularly important as there must be adequate airflow from the fan for removal of the substantial waste heat . or square . They sell these under a variety of specialty names. Replace other components in reverse order and then reattach the filament and HV wires. which tend to fail prematurely. a burnt aroma.
Replacing the magnetron
WARNING: First. a sharp blow or fall (during shipping as well if not properly packed) could shatter the filament. there is probably no glass in yours (unless it is quite old) so it isn't really very fragile. audible arcing. as well as under manufacturer brand names. the opposite can also be true. Transfer any thermal protector to the new unit.htm
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. The typical schematic is shown below:
http://www. When removing it from its mounting. do not lose the RF gasket . produce both high and low end magnetrons. and they can help you with proper choice and application. Usually. discharge the high voltage capacitor. Although the magnetron is a vacuum tube. Do keep it (the magnets) away from your diskettes unless you want them bulk erased! As for the old one. overheating. these after-market tubes are actually higher in quality than the original tube. or simply no heat. Here's the interesting thing.cone. bull nose. Make careful notes of both the wiring and mechanical relationships. Some manufacturers. Reuse it unless your replacement magnetron comes with a new one.Notes on the Troubleshooting and Repair of Microwave Ovens
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In my experience. as in the case of the OEM Sanyo magnetrons.repairfaq. QB products). It is critical that the replacement magnetron be mechanically identical: this means that the mounting configuration (studs or holes and their location). with power disconnected.

1 to .Notes on the Troubleshooting and Repair of Microwave Ovens
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+-------------------o White wire ||( Filament winding || +-------------------o White wire || || +-------------------o Red Wire AC H o---------+ ||( )||( )||( HV Winding . Testing the high voltage transformer more fully is difficult without fancy equipment. open windings (not very likely) can be located and other faults can be identified by the process of elimination. However. a winding-to-winding short would not cause enough of a resistance change to be detected with an ohmmeter unless you could compare with an identical model transformer from the same lot number. Only major short circuits can be identified in the transformer with an ohmmeter since the nominal resistance of the windings is unknown. you need to make sure that it hasn't been damaged as well. you can measure the primary current with all secondaries disconnected. Check the resistance between all windings (and to the core): Filament to primary.htm
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. high voltage.org/sam/micfaq. A typical midsize might be 65 ohms.5 ohms (. and similar fire resistant materials must be used for the repair as were present original.repairfaq.5 to 2. Therefore. However.2 ohms typical). Primary to high voltage and core should be infinite. Typical resistance readings for the transformer HV secondary are in the 25 to 150 ohms range (depending on the power rating of the oven) from HV connection to chassis.5 )||( 1.
http://www. An open would be an obvious failure.5 KV RMS ohms )||( . If you have a clamp-on ammeter. based on the way these are wound. See the section: Testing the HV transformer using an AC current meter.5 A or MORE )||( 25 to 150 ohms )||( AC N o---------+ ||( | +-+ HV return connected to frame | | AC G o------------+---+
Disconnect terminals as required to make the following tests: The resistances of the primary should be .1 to . if the filament winding is adjacent to the HV winding (in the same channel). and core. The resistance of the filament winding will likely be so low as not to be detectable with your multimeter. the arcing may have been taking place to the HV winding rather than the core. should be infinite. It may be possible to repair a filament winding which is shorted to the core (the only likely place) as it is only 2 or 3 turns of heavy wire. However. However. High voltage to core should be between 25 and 150 ohms as discussed above. it must be insulated for 5.000 V. The only measurement easily made would be that there is no short to the chassis. may get quite hot with normal use.

0 120 >4.3 90 . WARNING: Up to 3. discharge the HV capacitor.repairfaq. the transformer can be powered up to see if the primary current it draws is reasonable with no load. magnetron.000 VAC on HV terminal . If the fuse now blows. If the fuse still blows. you have a problem elsewhere such as a defective interlock or shorted wire. Check for damaged wires that may be shorting to the chassis.
Testing the HV transformer using an AC current meter
Where the HV transformer doesn't blow a fuse but overheats or produces insufficient output.1 110 2. Alternatively. Use a 3 prong cord with H and N connected to the primary and G firmly screwed to the transformer core/mounting structure. then the problem is likely with the triac (if used). just remove the 3 secondary connections and power it through the existing wiring using the normal oven controls.test out. Possibly something is failing only when full voltage is applied. and initiate a cook cycle. If you have a clamp-on AC ammeter. Assuming the fuse does not blow. If the fuse does not blow. the fuse may not actually blow (at least not immediately) but there will likely be a loud hum when the HV transformer is powered. Disconnect the primary of the HV transformer and initiate a cook cycle. remove the high voltage and filament connections to the transformer. Unplug the oven. the transformer is likely good and there are still problems in the high voltage components. magnetron . here are the input current readings at various input voltages for the HV transformer from a typical mid-size microwave oven:
Input VAC Input Amps -----------------------80 . unplug the oven and reconnect the primary of the HV transformer. If the fuse does not blow with the secondary isolated. However. a shorted wire.Notes on the Troubleshooting and Repair of Microwave Ovens
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Note: in the discussion below. then it is possible that the magnetron filament is shorted. HV capacitor. Also note that depending on the severity of the fault. reconnect only the magnetron filament (not the HV) to the transformer and power it up again. there may be a loud hum as the HV transformer struggles due to a fault in the HV transformer or a shorted HV diode. If the fuse still blows.org/sam/micfaq. If your oven uses a triac. or shorted transformer.htm
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. the problem is likely with the transformer. power up the oven.0
http://www. remove and bypass it.HV diode.6 100 1. Repair or replace these as necessary. Stand well clear when you apply power! Use of a Variac is recommended but not essential. Or. or a short in the HV wiring. If the other components . if the fuse still blows when the oven is plugged in (door closed to enable the interlocks). The meter's clamp needs to go around H or N but not both. it is assumed that the fuse is blowing due to a possible short in the HV transformer. Now.AND possibly other windings if there is a short in the transformer somewhere.0 115 3. this test may be useful.

with power disconnected.Notes on the Troubleshooting and Repair of Microwave Ovens
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Above about 100 VAC.you should have to work at removing them. If any connections between the lug and the wire or HV diode are at all loose. A bit of electrical tape may be all that is needed. Label the wires before pulling off the Fast-Ons if there is any doubt as to where they go. HV transformer. And.htm
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. If they are not too badly deteriorated. with power disconnected. discharge the high voltage capacitor. the transformer is likely good. See the section Safe discharging of the high voltage capacitor. Check for loose. solder it with a
http://www. However. or deteriorated lugs in the filament circuit (not just the magnetron). the non-linear increase in current indicates that the core is saturating with no load. Replacement of a HV transformer is straightforward but other assemblies may be using the transformer bolts for their mounting and/or may block your way. these readings do not indicate a problem. there was also a noticeable hum (though not nearly as great as with a secondary short). Shorted turns would result in much higher current at all input voltages. If the replacement transformer is not mechanically identical. discharge the high voltage capacitor. yes. Microwave oven transformers are designed with as little copper as possible.repairfaq. try squeezing them a little tighter with a pair of pliers and reinstall. the transformer must be secure . burnt. cut off the old ones and replace them. See the section Safe discharging of the high voltage capacitor.don't just sit it in place. No. they will still work even if they are somewhat ugly. weakening of the lug.org/sam/micfaq. Since the magnetron filament in particular uses high current.especially between the magnetron. They should not be loose .
Replacing the high voltage transformer
WARNING: First. and eventual failure or erratic operation. Try to pull off each of the lugs. and other components of the high voltage circuits for signs of arcing and excessive heating or burning. Arcing may be the result of the wire scraping against a sharp sheet metal edge due to poor placement and or vibration. Inspect the wiring . If the lugs and their wire connections appear to be in good condition but come off their terminals easily. any resistance at the press (Fast-On) connections will result in heating. you may need to use some creativity in anchoring it and any structures that are attached to its frame. more heating. Otherwise. Try not to drop either the old or new transformer on your foot!
Testing and repairing the wiring and connections
WARNING: First. note that some lugs are of the locking variety and require that you push a little tab to release them. If your readings are similar to these. If you find evidence of this: Remove the lugs and clean the terminals with fine sandpaper or a file. However.

there should never be any voltage across it unless there is actual overheating. use a drill to make a hole in each terminal. Replacement of a thermal protector is very straightforward as it is almost always screwed in place with push-on lug terminals.
Testing thermal protectors and thermal fuses
There may be two types of devices present in your oven: Thermal protectors are thermostats that open a set of high current contacts at a preset temperature. They blow and need to be replaced. The new thermal fuse will probably come with lugs attached. An overheating condition would generally be obvious as the mounting surface on which the thermal protector is located would be scorching hot when it tripped . Alternatively. If the bulb lights up or the meter indicates approximately line voltage . nut. and lockwasher. These approaches will work as long as there is enough metal remaining for a solid connection and may permit you to salvage a magnetron or HV transformer that would otherwise need to be replaced.too hot to touch (but discharge the HV capacitor first . Soldering is also possible. both types should read as a dead short with an ohmmeter (disconnect one terminal as there may be low resistance components or wiring which may confuse your readings). If the thermal protector is functioning properly. the device is bad. If the resistance is more than a small fraction of an ohm. clip a 100 W light bulb or AC voltmeter across it and operate the oven.org/sam/micfaq. If you suspect a bad thermal protector in the HV transformer primary. At room temperature. They should reset when they cool off. You must match both the temperature and current ratings. like a relay or switch.the thermal protector is defective and will need to be replaced. Thermal fuses will open at a preset temperature but do not reset.Notes on the Troubleshooting and Repair of Microwave Ovens
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high wattage soldering iron or soldering gun. and then fasten the (tinned) wire directly (or better yet) a new ring lug to the terminal with a machine screw. An open triac or one that didn't respond to the gate would result in no heat and possibly other things like the fan and turntable not working as well. scrape away the enamel and surface corrosion and resolder with a high wattage soldering iron or soldering gun.and there is no sign of overheating .repairfaq. However.
Testing and replacing the triac
A triac may fail in a variety of ways: A shorted triac would result in the oven coming on as soon as the door is closed or the power being stuck on high no matter what the touchpad setting. Also check for bad solder connections between the terminals on the high voltage transformer and the enameled wire used for its windings.
http://www. If you find anything suspect.htm
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. the contacts sometimes deteriorate. Replacements are somewhat readily available.a burn from the heat will be nothing compared to the potential shock!).

This will result in overheating as well as no or erratic operation. An open coil is obviously defective but sometimes the break is right at the terminal connections and can be repaired easily.htm
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. Nearly all triac failures will be shorts. triacs can fail in other . the use of contact cleaner first and a piece of paper pulled back and forth through the closed contacts may help. the relay may have an open coil.probably below the measurable limits on a typical multimeter . The relay will most likely need to be replaced if as in this case the contacts are switching any substantial power. test for voltage to the coil. If the voltage is correct. As noted above.a few milliohms. some of the windings are probably shorted.just don't get the wires mixed up. If the voltage is low or zero.possibly peculiar ways . worn. A triac where one half doesn't properly turn off would result in the main fuse blowing when the cook cycle completed. Replacement is very straightforward . A relay that doesn't close (due to defective contacts or a bad coil) would result in no heat and possibly other things like the fan and turntable not working as well. welded closed. the contacts are probably dirty.org/sam/micfaq. A few ohms means a bad triac.
Testing and replacing the power relay
A defective relay can result in a variety of symptoms: A relay with its contacts welded (stuck) closed would result in the oven coming on as soon as the door is closed or the power being stuck on high no matter what the touchpad setting. If the resistance is too low. If you measure significant or erratic resistance for the closed contacts as the relay is switched or if very gentle tapping results in erratic resistance changes. or there may be other mechanical problems. the coil may be shorted or the driving circuit may be defective. Superfine sandpaper may be used as a last resort but this is only a short term fix. If you can gain access by removing the cover. binding. measuring across the MT1 and MT2 terminals of the triac (the power connections) should read as a high resistance with a multimeter. If you can get at the contacts.Notes on the Troubleshooting and Repair of Microwave Ovens
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A triac that didn't turn off would result in the parts of the oven continuing to run even after the timer counted to zero. If the relay is totally inoperative. dirty. Remove the relay from the circuit (if possible) and measure the coil resistance. Compare your reading with the marked or specified value and/or compare with a known working relay of the same type. the contacts may be corroded.
http://www. a visual examination will confirm this.so substitution or bypassing may be necessary to rule out all possibilities. If the relay makes a normal switching sound but does not correctly control its output connections. or worn.repairfaq. Replacement will be required. Thus. corroded. The resistance of closed contacts on a relay that is in good condition should be very low .
Back to Microwave Oven Repair FAQ Table of Contents. A triac where one half was shorted would result in a blown fuse due to it acting as a rectifier pumping DC through the HV transformer.

They are usually made by the same companies that manufacture other service equipment.99 and an FDA approved unit (including calibration). The goal is to have a meter deflection of more than 10% of it's scale while not going off scale for sake of accuracy.) While I don't personally recall ever having damaged a probe while checking for leakage. An inexpensive meter is better than nothing but will not be as sensitive and will not allow you to quantify the amount of any leakage. They prevent radiant heat sources from affecting the meter reading. I have confirmed that by removing the styrofoam cone from the end of a Holaday uW leakage detector's probe and then bringing its tip near a heat source (40W bulb) caused the meter to have a significant deflection. I do know that it is possible to do so and did happen on rare occasions.htm
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.
Comments on microwave leakage meters
(From Barry Collins (bcollins@mindspring. While it didn't specifically mention damage to the probes. the cones are not only used as spacers. Newer designs (Holaday) claim to be more or less immune to damage resulting from placing them into high energy fields.) I found an old manual for a Narda 8100B Electromagnetic Leakage Monitor. Holaday (sp?) makes another line of detectors and those may use a thermistor array. The three probes were listed as (high/low range for each):
Probe Range ----------------------------------------8120A 0. for $388. as well. listen for alarm. If you work on microwave ovens.com). such a meter is a *must* for personal safety reasons as well as minimizing the risk of liability after returning them to your customers. Use it around the door seem and ventilation holes in the cabinet. MCM Electronics sells an inexpensive unit suitable for quick checks on a go/no-go basis for $6. Prices and capabilities vary widely. The Holaday probes that I used had 8 diodes in the tip that formed an array. The Narda manual states that their probes use an antenna/thermocouples design. (I used to work for a manufacturer of Microwave ovens...). etc. one should start with the highest power rated probe and work toward the lowest power rated probe (three listed in all). In a nutshell.repairfaq.org/sam/micfaq.Notes on the Troubleshooting and Repair of Microwave Ovens
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Items of Interest
Microwave leakage meters
A routine test for radiation leakage should be done before returning an oven you have worked on especially if the door or magnetron/waveguide were disturbed during the repair process. stop and replace probe. These should be available wherever you buy quality test instruments.0 mW/square cm
http://www. Note: you should also perform an electrical leakage test to assure that all case parts are securely connected to the Ground of the AC plug. there were overtones throughout the text that implied such (watch needle. There is a section in the Narda manual that details how to select the proper probe to measure "unknown" leakage levels. Thus. I do know that the older Narda equipment was prone to such damage.2 mW to 2.

slowly sweep the tester around the door seal. Damage would occur where one intentionally held the lower power rated probe in the strong field until the thermocouple (or thermistor?) overheated.org/sam/micfaq.ucar.000 watt plus models.0 mW to 20. Put a large container of water (>=2 cups) in the microwave and run it on HIGH for 2 minutes. It is simply a Schottky Barrier Diode (SBD) and an LED wired together.co. While it is running.0 mw/square cm in consumers home As you no doubt know. Determining by how much is why you pay the big bucks for a real leakage meter! WARNING: These are no substitute for a properly calibrated commercial unit! (From: Leon Heller (leon@lfheller. The leads of the SBD are left intact and straight and act as a 1/4 wavelength dipole. You may have to
http://www.0 mW to 200.) A very simple design I saw somewhere (Electronics World.) I then taped/glued it 1 1/2 and perpendicular from the end of a popsicle stick (this gives it a 'standoff' distance).demon. (From: Ren Tescher (ren@rap. You can buy these detectors quite cheaply.uk).) I saw an article about it in Modern Electronics in the early eighties. All microwave ovens leak to some extent.0 mw/square cm leaving the warehouse Less than 5. I think one measured the voltage across the diode via a resistor and capacitor smoothing arrangement using a 50 uA meter. hinges and door latch. Here's the circuit:
SBD <-----------------+-|<|-+-----------------> | | +-|>|-+ LED
The LED is soldered close to SBD using as short of leads as possible (being careful not to ruin either part with too much heat). (Note that the diodes are connected anode to cathode. their utility is somewhat limited. probably) consisted of a half-wave dipole with a Shottky diode detector between the two elements.htm
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.0 mW/square cm 20.sam). with a hole cut in the oven (in reference to those who want to modify one see the section: Microwave ovens for non-standard applications --.Notes on the Troubleshooting and Repair of Microwave Ovens
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8121A 8122A
2.edu).0 mw/square cm off of our assembly line Less than 3. but I believe that the maximum leakages we were allowed by the governmental agency were: Less than 2.repairfaq.0 mW/square cm
This is from memory.
Simple microwave leak detectors
Since these do not really provide an absolute measurement. the density can easily reach several times these numbers. especially on the newer 1. not cathode to cathode.

trash the oven. How safe is it to use (assuming you evicted the cockroaches)? As long as there is no serious damage to the door (a 6 inch hole would quality as serious damage) and the door fits square. Make sure the metal cover has all its fingers engaged around the front (though with a properly installed magnetron. The commercial tester had three ranges and the most sensitive range was divided into 3 color bands. there should be no leakage from there. Meyer (jimbob@acpub. You may even be able to use it to find wires behind drywall in your house.Notes on the Troubleshooting and Repair of Microwave Ovens
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dim the lights to see if the LED lights up.repairfaq. especially if the tester still glows if it is pulled beyond the 1-1/2 inch standoff distance to 3 inches. around the hinge/latch areas. or a thriving community of cockroaches inside. and when sufficient rectified voltage has built up.org/sam/micfaq. (From: James P. An inexpensive leakage tester . Typically the LED just flickers. The only problem you could find was a blown fuse. Any leaking uwaves will be picked up by the dipole 'antenna'. Make the divider network resistance large enough to limit the current through the bulb to just a couple of mA. there should be minimal microwave leakage into the electronics bay). (US law allows increased leakage as the oven ages). then you have located a leak.. the SBD will rectify the waves. However.) Get a small neon bulb. The home-built testers all 'fired' at some point in the 'yellow' range. the LED will light up. Adjust the voltage across the bulb so that it's just barely glowing. I built 10 of these at home and then compared them to the commercial tester we had at work.around $8 . Adding this onto a neon circuit tester is one option and will provide an insulated housing as well. Large leaks .they all came from the bargain bin at Radio shacks. Plug the whole thing into an AC outlet. (The screen really is a very good microwave shield --. red.
http://www.will not be as sensitive or accurate as the $500 variety by may provide some peace of mind. I attribute the variances within the yellow (caution) range to individual characteristics of the diodes . You may notice that no radiation leaks through the viewing window.htm
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. they may indicate dangerous leakage even when your oven is within acceptable limits. it should be properly sealed. Wave the bulb around the door gaskets and if it gets brighter when the oven is turned on. contrary to the old wives tale of not looking through the window while it's cooking.. The NE-2 size is a good one. Small leaks may be remedied by adjusting or cleaning the door and hinges and/or by distance (square law= doubling the distance quarters the power). Use some resistors to make a voltage divider for 115 VAC to feed the bulb.duke. As long as the waveguide is tightly mounted and undamaged. as noted below.
How safe is a repaired microwave oven?
So you fixed up Aunt Minnie's Radarange or picked up a microwave at a yard sale or scavenged one off the curb..edu).sam). INSULATE everything completely. A solid glow would indicate excessive leakage. green. truly horrible mess of decayed burnt-on food. The bulb detector can be very sensitive. The most important considerations are the door and door seal. Put the bulb on the end of a line cord and plug. yellow.

Furthermore. CO detectors caused similar panic among users of the appliances. In addition. The windings AWG got smaller and the temperature rise went up accordingly. shape.
http://www. Size was reduced and the number of fins were reduced.00 instead of $9. etc. the microwave oven may use 1/10th the energy of a typical electric cooktop element to bring it to a boil! Therefore. non-cost reduced.even if you could distort its anatomy enough to fit the typical mid-size microwave!
Microwave oven design and cost reduction
(From Barry Collins (bcollins@mindspring. you won't heat just *1 cup* of tea but more likely 2 or 3 just to be sure you have enough! A microwave oven is not likely to be more than 60% efficient . using an electric stove to heat 1 cup of tea may result in much wasted energy as the element and pot must be heated as well and there are losses due to convection and conduction to the surrounding environment.com). placement of food items affect the SWR. The magnetrons were cost reduced in a similar fashion.) Microwave oven design is a black art. These all add up to a significant overhead. The microwaves are designed for the most part to work optimally with an average load.com). they have caused a more than a few people to unnecessarily fear microwave ovens over the years.
Efficiency of microwave ovens
The efficiency of an electric heating element is 100% .) can give very inaccurate readings. gastronomic preferences aside.45). Models equipped with turn-table models compensate for this by breaking up the SWR as the food revolves.Notes on the Troubleshooting and Repair of Microwave Ovens
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(From Barry Collins (bcollins@mindspring.) Those inexpensive hand held meters (from Radio Shack. I personally know that it had one of the lowest SWRs available at the time. For a single cup of tea. But then the cost went from $300 to $149 while life went from 10 years-plus to 5 years or less and they became disposable items. there are losses in the high voltage transformer. However. and turntable motor (if used). However. Size. Their temperature went up while their efficiency went down. The light bulb and controller also use small amounts of power. I just changed jobs from working for a company that made gas ranges. a conventional oven is better suited for that 20 pound turkey . What one hopes for is to deliver all the power from the magnetron into the food and not have a high SWR reflect back into the magnetron and burn it out. it makes sense to use a microwave oven for small short tasks where the losses of an electric or gas oven or cooktop would dominate. I'd almost hesitate to hope the Government would have mandated an efficiency. My oven has a stirrer fan design and has been working for going on 18 years now without the first hint of a problem (maybe a little less power). That's one area. While the magnetron tube itself may have an efficiency rating of 75%. However. you are not heating the surrounding countryside as the microwaves only affects what you are cooking and not the container or oven cavity itself and you are more likely to only load the amount of food you expect to be eating.period. While they definitely serve a purpose. cooler running. more efficient magnetron (that cost $13. The thing that I found disturbing about microwave oven design was the trends to go with hotter an hotter insulation classes on the components used in them.repairfaq.possibly as low as 50 percent or even less. the waveform applied to the magnetron by the half wave doubler circuit is not ideal for maximum efficiency. I'd highly recommend anyone with gas heat or appliances to purchase a quality CO detector. The original transformers were class H while the newer ones are now class N. Also. but not one of those inexpensive type that go off whenever there is a thermal inversion of smog a city.htm
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.org/sam/micfaq. Not to mention it has an older design. cooling fans. This was all done in the name of cost reduction to remain competitive..

but any metal lid on a bottle largely full of microwave-absorbing stuff should not present a problem especially if the bottle is on its side so that stuff is contacting or very nearly contacting much of the lid. you need exposed water or food to absorb the microwaves.) from the walls to avoid arcing. If you have exposed food or water. This is usually OK.repairfaq. the magnetron should be OK. In the event the microwave runs empty OK.g. and in an unpredictable manner.com). Safe distances are uncertain and are usually less if the metal objects are small and a large amount of food or water is exposed. If you add metal objects in a manner safe for the tube. Any arcing is generally not a good thing. They don't need preheating! :)
More on metal in the microwave
(From: Don Klipstein (don@Misty. they just reflect around the oven and get back to the magnetron tube. Having absolutely nothing in the oven chamber or just metal is the potentially more likely damaging situation for the magnetron as you are dumping several hundred W to over a kW of power into a reflective cavity with no load. It is even not too good to run a microwave empty. A plain glass bottle if ice-cold stuff might possibly break from thermal shock when heated.htm
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.Notes on the Troubleshooting and Repair of Microwave Ovens
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Problems with running a microwave oven with metal inside or totally empty
Metal in microwave ovens may or may not be a problem depending on the specific situation. arc. If any metal object has major contact with a microwave absorbing food target and such target is still heavily exposed. In the worst case. Examples would be wrapping foil around the wingtips of a whole chicken or whole turkey. the tube should not mind some stray metal too much. Older microwave ovens with used glass magnetrons were perhaps more susceptible to these disasters (all modern overs use magnetrons with ceramic construction but I really don't know how much this matters) but it's still a good idea to avoid running a microwave empty. The walls of the main cooking chamber are metal.org/sam/micfaq. try to keep these at lease a half inch (a bit over a cm. adding metal objects change the microwave reflection pattern and might possibly unfavorably change things. Sharp edges and points create strong field gradients which tend to spark. Sharp convection/microwave combo). You will note that some ovens come with metal fixtures in addition to the oven walls themselves (e. Metal objects close to other metal objects or to the walls of the cooking chamber may arc to these. this is probably not likely to damage the oven. Just avoid unrelated problems due to major temperature change of anything in contact with a non-heat-rated glass container. This may be bad for the tube. or create other fireworks. or a bottle of liquid (on its side) with a metal lid with liquid contacting much of the lid. there is another concern. you should be OK. you could end up with a meltdown inside the waveguide requiring replacement of various expensive components including the magnetron.. With some food in the oven to absorb the power. Otherwise. Even if the tube does not mind. If the added metal does not interfere with microwaves mainly getting from the tube to the target food or water and being absorbed.after incident
http://www.
Burnt smell from oven .) Mainly.

A GFCI detects any difference between the currents in the Hot and Neutral wires and shuts off the power should this difference exceed a few mA. but the lasting effect is that there is a strong odor. Check with a circuit tester to make sure your 3 prong outlet is correctly wired. with the case floating. If this doesn't help enough. At least. nothing in the oven has actually sustained any damage.or at random times. The rest is wasted as heat. nuisance tripping of the GFCI may occur when you attempt to cook anything . A GFCI is not needed with a properly grounded microwave oven as any such fault will blow a fuse or trip a circuit breaker. I would expect that the smell will decrease and eventually go away. remove the waveguide cover and clean it and as best as possible the accessible part of the waveguide. Only about 50 to 60% of the electricity used by a microwave oven actually gets turned into microwaves. Touching this may result in a shock or worse.
Can a microwave oven be built into (or hung under) a cabinet?
http://www. Install one if it is not grounded. In most cases. this will at most blow a fuse.org/sam/micfaq.nearly an entire 15 Amp circuit. It won't hurt . with some combinations of oven design and your particular wiring. it will not hurt to have a GFCI as well. However. there are many passages where the air would normally circulate in convection mode which will be coated even if the oven was used in microwave mode. do not put your refrigerator on the same circuit!
Microwave ovens and GFCIs
A Ground Fault Circuit Interrupter (GFCI) protects people from shocks should a situation develop where an accessible part of an appliance should short to a live wire.especially considering that microwave ovens are usually situated near grounded appliances like ranges and normal ovens and wet areas like kitchen sinks.repairfaq. Plug the oven into a properly grounded circuit not on a GFCI. I wasn't home at the time. it is theoretically possible for the entire high voltage to appear on the metal case should certain internal connections come loose. smoke may have gotten into the waveguide above the oven chamber." Start by cleaning the interior of the oven thoroughly with mild detergent and water. What do you recommend. However. a 700 W oven will actually use up to 1400 W of power . While unlikely. However. Some have suggested boiling a cup of lemon scented water or vinegar to help speed things along. With a properly grounded outlet. There is a very important safety reason for this requirement: the return for the high voltage is through the chassis. I have a Sharp Convection/Microwave. A dedicated circuit is desirable since microwave ovens are significant users of power. due to the highly inductive nature of the high voltage transformer.maybe even help. You may have to do this several times to get all of the sticky film left behind. so I don't know if it was neglect or inappropriate use. Many are not. However. similar to that which you smell after a fire that I cannot seem to get rid of. this usually does not indicate any problem. Therefore.htm
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. :)
Microwave ovens and grounded dedicated circuits
A microwave oven should be used only on a properly wired 3 wire grounded circuit. Convection ovens have heating elements which are similar energy hogs.Notes on the Troubleshooting and Repair of Microwave Ovens
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"My daughter tried to heat up one of those 'soup in a box' containers and it burned actually charred. Most likely. that even after the incident described still performs well. the odor may persist since the smoke can penetrate to places you cannot access for cleaning. However. If possible. With a combination convection and microwave oven especially. a shocking (or worse) situation could develop .

http://www.edu). wherever you can find studs.cabinetry and/or drywall may not be up to the task.the microwave weighs quite a bit and must endure a fair amount of abuse from heavy casseroles and the inevitable door yanking/slamming! Note that one of the advantages of buying a microwave oven designed for under cabinet or wall mounting is that it may provide convenient access for servicing from the front . So. screws. :-( These are HEAVY appliances . which depends on line frequency. There is a backplate which you attach to the wall with whatever combination of lag bolts. expansion bolts. line frequency may make a difference. Low cost transformers or international voltage adapters will not work.) I've installed a GE over-the-range microwave. You then bolt it into the cabinet above it for additional security. my recommendation would be to build a shelf rather than a totally sealed. conformal cabinet. the HV capacitor is in series with the magnetron and thus its impedance. The rear-bottom edge of the oven then clips onto the backplate to form a kind of hinge.remove a couple of screws and most of the internal components can be accessed for service. check with your friendly township inspector! There are special (likely highly overpriced) models available for this type of mounting.
Taking a microwave oven oversees (or vice versa)
Microwave ovens are high power appliances. the major issues are: Providing adequate air flow through its ventilation grill which is usually located in the rear.nyu.not having to remove the entire unit to check or change a fuse! For example. Just make sure it is securely supported . Sell the oven before you leave and buy a new one at your destination. Models designed as over-the-range or combined microwave and exhaust fan units mount via a massive plate fastened securely into the wall structure (screwed directly to the studs. (From: Roy Smith (roy@popmail. You will need a heavy and expensive step down or step up transformer which will likely cost as much as a new microwave oven. (A convection/microwave can get quite hot and have ventilation in other places. It can have sides and a top as long as you leave a couple of inches all around.med. It comes with a template to make this easy.htm
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. Local building codes may specify when and if this approach can be used.repairfaq.e. There are two long bolts that run the depth of the oven near the top which you use to complete the attachment of the oven to the backplate. Due to the way the high voltage power supply works in a microwave oven. To use a normal microwave. They may additionally be bolted into the cabinet above but this will not (or should not) be the sole means of support. for microwave ovens in particular. In this case I would suggest contacting the manufacturer of the oven for specific requirements. etc you can get to work (i. It really was quite straight-forward. Furthermore. This would not be possible where a countertop oven is used in a permanent installation. affects output power.) Providing adequate structural support so the microwave doesn't end up in the soup.org/sam/micfaq. etc). enclosed. This will result in a microwave oven that is much more easily serviced should the need arise and replaced in the future with a model that is not quite identical. and you pivot the oven up into place. before doing any demolition.Notes on the Troubleshooting and Repair of Microwave Ovens
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Assuming it is a regular microwave and not a convection/microwave combo. some GE units have a hinged front panel . not just the sheetrock!).

though this is not likely. Using a slightly lower line voltage will reduce the heating but will further decrease the cooking power. Going from 60 Hz to 50 Hz may slightly decrease output power and possibly increase heating of the HV transformer due to core losses.htm
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.net).2 _|_ (Except for filament.org/sam/micfaq. at the same or higher operating voltage. these may run hot even at the correct line frequency of 60 Hz. See High Voltage Inverter Power Supply from Sharp Microwave Oven.
High frequency inverter type HV power supplies
While the vast majority of microwave ovens .018 uF | | \/ | N o----+---|<|---+ Drive |/ C ::( 2. Alternatively.perhaps terminally .use minor variations on the tried and trusted half wave doubler circuit.repairfaq. So going to 50 Hz would make it worse .Notes on the Troubleshooting and Repair of Microwave Ovens
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High voltage transformer core saturation may also be a problem. Check the manual for any self-test info. This self-test is usually accessed by pressing a couple of keys on the touch pad. # turns estimated) o H1 .in many ways similar to the deflection/HV flyback power supply of a TV or monitor.perhaps every single one you will ever see . You can usually test things like keys. switches controller etc. This means that the microwave output is pulsing at both 60 Hz and the frequency of the inverter!
Inverter Transformer Magnetron o H o----+---|>|------+--------+-------+ +--------------------------+ ~| |+ _|_ Drive )::( Filament 1T #18 | +---|<|---+ | --25T ):: +--------------+------+ | 115 VAC | | | #12 ):: HV Cap | +-|----|-+ +---|>|---|--+ +-------+ :: +-------||-----+ | |_ _| | | | | ::( . Even with no load. Some microwaves have this information tucked in a pocket or hidden somewhere behind panels.Chassis Ground Bridge Rectifier
http://www. The line voltage could be reduced by a small amount to compensate. this may partially make up for your change in output power! :-)
Microwave oven test-mode
(From Mark Paladino (paladino@frontiernet. Going from 50 Hz to 60 Hz at the same line voltage may slightly increase output cooking power (and heating of the magnetron). The digital clock and timer will likely run slow or fast if the line frequency changes as they usually use the power line for reference. a few models have been designed using solid state high frequency inverters . Of course. A typical circuit (from a Sharp microwave oven) uses full wave rectified but mostly unfiltered pulsating DC as the power to a large ferrite inverter transformer which sort of looks like a flyback on steroids. This is best done with a buck/boost transformer rated for the maximum current input to the microwave oven (usually 15 A).400 V __|__ | ___ | ~ |o---| Chopper ::( HV _\_/_ +----|:--+ (Interlocks and | |\ E ::( 250T | HV |'--> fuses/protectors | | ::( #26 Sense | diode | uWaves not shown) +-----------+ +--+---/\/\----+---------+ o | 1. it may be possible to replace the HV capacitor with one that has about 5/6 the uF value.) Some microwave ovens have a self-test feature.

See the
http://www. Possibly more have jumped off. It is a LARGE NPN type on a LARGE heatsink. QM50HJ-H. I don't know if any other manufacturers (including Sharp) still do. relay and/or triac control of the AC power. heavy transformers can smash your feet and sharp sheet metal can cut flesh. the magnets in the magnetron may erase your diskettes or mess up the colors on your TV. These usually operate on 115 VAC but some may use low voltage DC. consider that many of the parts are interchangeable and may be useful should your *new* oven ever need repair! For the hobbiest. Of course. component level troubleshooting and repair would be too time consuming. With appropriate supervision. However. Except for obvious problems like a tired fuse. :)
Dangerous (or useful) parts in a dead microwave oven?
A microwave oven with its power cord cut or removed AND its high voltage capacitor safely discharged is an inanimate object. an investigation of the inside of a deceased microwave oven can be very interesting.is somewhat irrelevant in a microwave oven.cooling fan and turntable (if used). before you cannibalize your old oven. other companies certainly aren't jumping on the bandwagon. Compared to the simplicity of the common half wave doubler. They can easily be adapted to other uses.Notes on the Troubleshooting and Repair of Microwave Ovens
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The chopper transistor is marked: Mitsubishi. Perhaps. there are. :-) Note the similarity between the normal half wave doubler circuit and this output configuration! Base drive to the chopper transistor is provided by some relatively complex control circuitry using two additional sets of windings on the inverter transformer (not shown) for feedback and other functions in addition to current monitoring via the 'Sense' resistor in the transformer return. Some may feel there is nothing of interest inside a microwave oven. or by pulse width modulation of the high frequency power. Controller and touchpad . in fact.htm
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. it isn't at all surprising why these never caught on (what is diagramed above includes perhaps 1/10th the actual number of components in a typical inverter module. as with a switchmode power supply (which is what these really are) there could be multiple faults which would result in immediate failure or long term reliability problems if all bad parts were not located. And.digital timer. Furthermore. the blurb for the current line of Panasonic Genius(tm) inverter microwave ovens does boast about providing actual power continuously at each setting. which can be seen in the photo). I acquired the Sharp unit at least 5 years ago. this was yet another situation where the Marketing department needed something new and improved! But if it was a "must have". There are no particularly hazardous parts inside. 01AA2. possibly 8 years ago (that would be in 1996). some useful devices inside: Motors . I would counter that anything unfamiliar can be of immense educational value to children of all ages.org/sam/micfaq. The microwave distribution mechanism is at least as important in this regard. Schematics are not likely available either. a replacement module would likely cost as much as a new oven! This may simply be a situation where a high tech solution might not have been the best approach. The high frequency inverter approach would not seem to provide any important benefits in terms of functionality or efficiency yet created many more possible opportunities for failure. Panasonic has a several models like this. However. It is not known whether power levels in the oven from which this particular inverter unit came were set by the normal long cycle pulse width modulation or by control over a much shorter time scale. Another major advantage reduced weight .repairfaq. The principle advantages claimed by the manufacturer are more even cooking and less overcooking of edges. And.

You don't need to get inside to remove the magnets. Dangerous microwave leakage is possible. other miscellaneous parts.org/sam/micfaq. and HV capacitor (approximately 1 uF.) Spread the frame apart just a bit and lift out the tube with heat sink fins. CAUTION: the sheet metal fins may be sharp! The magnets can now be pulled off. diskettes.3 or more high current microswitches. up to 1. High voltage components (VERY DANGEROUS if powered) . mechanical wrist watches. The ferrite is
http://www. There is no residual radiation but it does contains a pair of powerful ferrite ring magnets. This usually requires peeling off the sheet metal around the edges.500 to 2.500 VRMS. DOUBLE WARNING: Do not even think about powering the magnetron once you have removed any parts or altered anything mechanical in the oven. :-( The magnetron tube itself can be disassembled by grinding off the welds around the edges of the large cylinder or cutting around it outer edge near one end with a hack saw but it takes quite a bit of curiosity to make this a worthwhile exercise. Keep the magnets a safe distance away from any magnetic media including what might be in your back pocket. Magnetron . Remove the cover over the box where the filament connections are located.500 to 2. 0. HV rectifier (12 to 15 kV PRV. Take appropriate precautions to protect your credit cards. and color computer monitors and TVs.5 A). Paint the magnets with plastic enamel or coat them with the stuff used on tool handles to reduce their tendency to chip. and mechanical wristwatches. They may need cleaning.Typical HV transformer (1. Interlock switches . diskettes.there are some nifty powerful magnets as part of the assembly. Cut the thick copper connections to the filament near the tube itself.5 A). See the section: The magnets in dead magnetrons. fuse holder. (The thick copper coils are RFI chokes and prevent any microwave energy from escaping via the filament circuit. 0. and credit cards.200 to 7.
The magnets in dead magnetrons
The dead magnetron you just replaced is fairly harmless. The chips are as magnetic as the overall magnet. If you do want to save the magnets: Disassemble the magnetron assembly as follows: Remove the top portion of the magnetron . they are that powerful) but they will suck all the bits right off your tapes.Notes on the Troubleshooting and Repair of Microwave Ovens
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section: Using the control panel from defunct microwave oven as an electronic timer. There is a slight chance that the coating on the filament is poisonous so don't take chances.htm
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.repairfaq.it is either fastened with screws or some metal tabs which are easily bent out of the way. Not only can they pinch flesh (yes. thermal protector.500 VAC (4. These can be removed without extensive disassembly and make really nice toys but should be handled with care.000 VDC). Heavy duty power cord.

Just cycling faster (without any other modifications is not the answer). This question was posed by someone who wanted to modify the circuitry to their microwave oven to provide continuous control and a constant heating rate. However. My guess is that this would be between 60 and 80 percent and full voltage from the Variac will result in 0 to 100 percent of cooking power (the magnetron is a non-linear device .
Using the control panel from defunct microwave oven as an electronic timer
It is usually possible to remove just the touchpad and controller board to use as a stand-alone timer with a switched output.a high power light dimmer or motor speed control might even work. Also see the section: Magnetron construction . this results in unsatisfactory results. a triac or solid state relay can be turned on and off at the peaks of the AC (to minimize inrush) similar to the pulse width modulation that is normally used for the oven .but at a much higher frequency. Be careful when disconnecting the touchpanel as the printed flex cable is fragile. for other purposes. If it uses a triac. and make sure the cooling fan is always powered from the full line voltage.repairfaq.between the controller and HV primary.) The power to the filament will still be affected but there will be a range over which continuous control will be possible. Note that power on a microwave oven is regulated by slow pulse width modulation . these magnets can be used to demonstrate many fascinating principles of magnetism. Store the magnets in a box packed in the center of another box with at least 4 inches on all sides. Clearly mark: powerful magnets with appropriate warnings. If the filament were put on its own transformer (with appropriate insulation ratings). DON'T attach it externally. the 10 to 30 second cycle time typically used for microwave oven pulse width heat control is fine. Having said that.Notes on the Troubleshooting and Repair of Microwave Ovens
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basically a ceramic and fragile. this approach may be the adequate. there will be a lag as the filament heats and cools. (For safety. The output will control a 10-15 A AC load using its built in relay or triac (though these may be mounted separately in the oven). With many models. the triac is NOT phase angle controlled . It should be possible to put a Variac (variable autotransformer) on the input to the high voltage transformer . Take care not to get your skin between the magnets when you bring them together since the attractive force when nearly touching is substantial. Smack them too hard and they will shatter.there is a threshold voltage below which no output is generated). This would result in a very non-linear relationship between on-time and power as the cycle became shorter and shorter. then instantaneous control of power should be possible using a Variac on the HV transformer primary or a phase control scheme using a triac .just switched on or off. Alternatively. the touchpanel (membrane touchpad) needs to be peeled off of the front plastic panel or the entire assembly can be removed intact.modern microwave oven. DON'T bypass or disable any door interlocks. However.htm
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.org/sam/micfaq. One problem is that the filament of the magnetron is turned on and off as well. Where manual control is all that is needed.
Precise control of microwave oven power
For heating a casserole. Have fun but be careful. This could easily be computer controlled with feedback from a temperature
http://www.order of a 30 second cycle if this matters.

to be on the full line voltage not affected by any power control scheme or timer. close the door hit start and it continues until it's time for you to do some thing else.net). There is even an 'on-line' help feature. and it sets itself up and takes off. etc.Notes on the Troubleshooting and Repair of Microwave Ovens
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sensor. In any case. They looked just way too busy. It will turn on for a while. While that sounds like overkill." "frozen dinner." "hot dog. Cook.sam." etc. hit start and it picks the time and power level. and includes 600 recipes (!). does it run Lotus??? :-) --. cover the edges.including cooling fans .com). choose a number from the list. It even has the obligatory POPCORN button! Another neat feature is that you can hold the start button on without setting any time and it will stay on for as long as you hold the button.) The one we bought has an LCD screen that's maybe three inches square. For things like CompuDefrost. how many pounds. There is a list of common items that you might Defrost.repairfaq. But. Same idea for CompuReHeat: Tell it how many slices of pizza or bowls of pasta you want to reheat. You pick one of those tasks. etc. then beep at you and tell you to break the pieces apart. You WILL see ovens (if they don't exist already) that with the help of a barcode or Dallas ID chip on the frozen package or food container. the attraction for me was that the menu-driven interface actually seemed simpler and more inviting than the ovens with timing buttons and 24 others marked "popcorn. Enter Quantity Press Start
Unit turns on and starts cooking. You do as you are told.) We bought a Sharp unit with the Interactive Display feature. takes you step-by-step through anything the oven can do. If the little word HELP lights up. DO NOT COVER. will contact a recipe database at the Web site for the product to determine exactly how to optimally overcook it and turn it into rubber. (From: Steve Dropkin (sdropkin@isd.
Has technology gone too far?
Don't you just hate it when your kitchen appliances have the highest IQ in the household? What more could you want? Maybe." "waffles. you tell it what you are defrosting. or CUT IN HALF.kodak." "beverage." "baked potato. enter the 'quantity'.org/sam/micfaq. and hit start. you press the HELP button and it gives you little hints like. This is great for melting cheese. etc." "sandwich. :) (From: Dave Marulli (marulli@rdcs. or Reheat. (Same argument I
http://www. A typical session goes like this:
Button Pressed ---------------CompuCook 1 4 Screen Output -----------------------------Enter Food Category Baked Potato. you want everything else . you haven't seen it all.htm
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. softening butter or chocolate. a microwave with a robot arm to retrieve the food from your fridge or freezer! But wait. Just the the World needs is a smart microwave.

I am concerned about potential microwave leakage of a harmful nature. The operators may have to remain further away or some additional shields may needed if these distances are not satisfactory. for example) to keep the magnetron happy. Obviously. these have to be taken on a case-by-case basis. The leakage detector or microwave field strength meter should come with information on acceptable
http://www. If any modifications are made to the oven that would compromise the integrity of the door seals or provide other places where microwave radiation could escape. But that popcorn button sure is a good selling point! :)
Microwave ovens for non-standard applications
Occasionally.heating is not as uniform as with normal ovens. softening sticks of Dynamite is probably not to be recommended! (There actually is a reason for this . A large appliance repair shop or electronics store may rent you one if you are persistent enough.htm
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. The manual says to monitor the popcorn anyway since it varies based on bag size.org/sam/micfaq. The smaller the opening. So on principal I choose 5 minutes on high and stop it at 1:45 (why not set for 3:15? because the one time I tried it the popcorn was burnt!). the less will be the leakage." Geez!!! You guys are out of your collective mind. use a good conductive sheet metal such as copper or aluminum to reduce the size of the opening as close to the material as possible. The wood stock will tend to reduce leakage while it is in place but the opening will leak like crazy when there is nothing in the hole. As far as I can tell.a microwave can develop hot spots . Next. 5) and popcorn with two presses (POPCORN. They consist of a ceramic (expanded alumina or something similar) insulating cylinder lined with a microwave susceptor . Temperatures exceeding 1000 degrees C (yellow-white heat) are possible after a few minutes on high.. having said that I feel much better. I just want to listen to the music. Use this to identify the safe limits front and back. However. all it does is automatically set 5 minutes. if you insist. :-( My first recommendation (though this is too weak a term) would to not do this.Notes on the Troubleshooting and Repair of Microwave Ovens
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have against a lot of mainstream HiFi equipment these days. Sorry. people ask questions about the use of a microwave oven to do things other than heating food. My second (and up to N where N is a very large number) recommendation would be not to do this. I can choose 5 minutes with two presses (QUICK.ca).) Our microwave has a button for popcorn. START). Special kilns that will fit inside a microwave oven are apparently available to achieve really high temperatures. etc. In general. borrow an accurate microwave leakage detector.) (From: Andrew Webber (webbers@magma.. The following is one such case in point: "My Dad and I are using a microwave oven to heat oak strips by passing them through the microwave field of a 1000W oven. not reengineer the sound source . Label these and don't go closer while the oven is in operation.possibly a ferrite material. Do your dynamite softening in a normal oven).repairfaq. Also. make sure there is always a load in the oven (a cup of water. See for example Microwave Melting of Metals. then special tests MUST be done to assure the safety of the users of the equipment. We cut out squares (4"x 4") in the glass front and metal back of the oven to allow these strips to pass through the field. The sheet metal must be in electrical contact with the mesh in the door and the metal back.

It will soon be back unless all they do is heat coffee. CAUTION (In addition to the loony nature of this entire project!): Since the leakage you encounter may be orders of magnitude greater than what is typical of even a misaligned microwave oven. provided they had taken reasonable precautions (too numerous to list). This forces air over the stirrer when the cover is replaced. The aqueous membranes of the eyes are perfect absorption material for stray microwaves.. It is something like 2 mW per square cm a foot or so from the oven . If stirrer still does not turn.htm
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.org/sam/micfaq. there is no assurance that even this limit is safe. However. (I met one of the Japanese engineers who had unintentionally placed his head in a test oven that was working. Never let one go out of the shop unless the stirrer is turning.e. start with the probe at a distance of a few feet and slowly move it closer while watching the meter or readout. Power does fall off with the square of the distance and microwaves. If this is ok or you correct it and stirrer still does not turn. The only known confirmed danger from microwave radiation is from internal heating effects.check it out.com). If your blower is running up to speed. Thus. The human body does not have an adequate warning system since nerve endings sensitive to heat are somewhat sparse. If the stirrer does not turn. Next time it may be a cavity or magnetron overload that has opened due to the stirrer not turning. remove the cover and replace the foam gasket material. The eye is particularly sensitive to this and it doesn't take much of an increase in temperature to denature the tissue of the central nervous system (i. In addition the stirrer bearing will sometimes arc and may melt at the spots where it arcs. but has one small modification which you will see when you compare the two. The Narda manual has multiple warning in it about this.net). scramble your brain). it doesn't make sense to take chances. aside from the resulting joke. Don't set it next the opening as you hit START! This will prevent the possibility of damage to the expensive leakage tester (which could be costly) and exposure risk to you as well.
http://www.Notes on the Troubleshooting and Repair of Microwave Ovens
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power limits. remove the grease shield and check the stirrer for burns that are causing it to stick.
Short course on Amana
(From: Charles Godard (cgodard@iamerica. Just don't stand in front of the source. What is wrong with radiant heat??? (From Barry Collins (bcollins@mindspring. He reported warmth.repairfaq. barring any reflective surface. but no lasting damage.) You did the right thing to discourage people from breaching the integrity of a microwave oven. I personally don't feel uncomfortable with what the person was doing.) Field density and exposure time is a large factor. because there are so many factors involved that one has to assume personal (or property) injury (or damage) may result from such actions. while the dangers may be overstated. I think the story goes that this was how the heating affect was originally discovered.) Everything depends on "Air Flow". It is this that you might want to point out in your FAQ's. One tends to remove one's hand when one senses heat. are very directional by nature. The number one precaution I've always held near and dear to me is to protect one's eyes. then replace the grease shield with a later model that looks almost the same as the original. you will always get a "Hot! spot" on the left bottom of the door. This can happen much faster than with fleshy parts of the body and don't heal anywhere near the way a flesh injury does.

I wish I had a hundred restaurant customers using them. The older Amana's power stays near 1500 watts forever. fuses. a service manual is rarely required. But if the contents of the microwavesafe container are being heated. I have gotten conflicting answers from the guy who rebuilt my computer and the guys at Radio Shack. you will need separate grounded electrical circuits for the microwave and computer equipment if you intend to ever use them at the same time. The problem would more likely be the magnetic field from the large transformer in the microwave oven causing interference on your monitor (wiggling. No adjustments or alignment should even be required so detailed procedures for these are not needed.
Why Microwave-Safe Containers Get Destroyed
You probably have a cabinet full of so-called microwave-safe containers that look like they have been exposed to damage from a nuclear explosion. :)
Back to Microwave Oven Repair FAQ Table of Contents. when tackling electronic faults in the controller. Why? It probably comes down to unequal heating of the contents or heating continuing long past the point where boiling takes place. Unlike most other types of consumer electronic equipment. My feeling is that it should be fine. some
http://www." Did the kids at Radio Shack even understand the question??? :) Your request is certainly a bit unusual. jiggling.to $125 repair bills. then some parts will get much hotter than others resulting in local melting and other damage. There should be no significant microwave leakage from the oven. especially the rear. shimmering.repairfaq. A sufficiently detailed schematic is nearly always pasted to the inside of the cover and includes all power components.org/sam/micfaq. For legal reasons. you still have some options other than surrendering your microwave to the local service center or the dumpster. interlocks. due to its effect on the electron beams in the CRT).htm
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. However.
Service Information
Advanced troubleshooting
If the solutions to your problems have not been covered in this document. What a waste!
Computer system near microwave oven?
"Can placing my microwave oven in close proximity to my computer and printer do any damage to either of them? The back of the oven would be right next to the printer and about 16 inches from the computer. I would assume that putting a microwave-safe container in an oven with a cup of water in a separate container wouldn't result in any damage to the microwave-safe container. protectors. a service manual with schematics will prove essential. Whether these are available depends on the manufacturer. Keep in mind that there is a computer of sorts inside the microwave controlling it! However. This is entirely sufficient to deal with any problems in the microwave generator. I doubt it is the microwave radiation itself doing anything to the material of the container directly and complaining to the oven manufacturer isn't likely to be very satisfying. Retail customers are junking them because of $100 . etc. and wiring.Notes on the Troubleshooting and Repair of Microwave Ovens
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It's good work on a quality product.

they won't be cheap .00 (unless original replacement in which case you will need to take out a mortgage . Power fuse: $. ISBN 0-8306-3457-6 (pbk. IA. PA 17294-0850 ISBN 0-8306-6457-2 (hard). Parts suppliers like MCM Electronics can provide these components to fit the vast majority of microwave ovens. Davidson TAB Books. Microwave Oven Repair. Common generic replacements are $30-40.org/sam/micfaq.htm
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. However.) This may be available at your public library (621. Overtemperature thermostat (thermal protector): $4. 2nd Edition Homer L.. 1991 Blue Ridge Summit.83 or 683.50. While the HV transformers are fairly standard. It should be possible to replace these with the $2 variety with wire leads).50. Eagle Grove.a touchpad may cost $30 or more. MCM Electronics now lists at least one Goldstar model replacement. Prices are high . Magnetron: $30-100.83 if your library is numbered that way) or from a technical bookstore. There is a good chance that your specific problem is covered.
http://www. However.try the generic variety).expect to pay $50 or more!!! In addition. the question is whether an oven that is a few years old is worth fixing. a division of McGraw Hill. either.
Cost of repair parts
Assuming you have located one or more bad components. Triac: $12.40. It is very complete and includes many actual repair case histories. 1.Notes on the Troubleshooting and Repair of Microwave Ovens
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manufacturers are reluctant to sell service information or replacement parts for microwave ovens. They are concerned with litigation should an unqualified person be injured or killed. Inc. Interlock Switch: $2. Here is one place that seems to stock some: AMI Parts.
Suggested Reference
I know of at least one book dealing specifically with microwave oven repair. Typical parts cost for generic replacements: HV diode: $2-5 (except for the bolt-on variety which can range up to $50. they do not fail that often. Voice phone: 1-800-5221264. Sensors and other manufacturer specific parts will be expensive.repairfaq. Touchpads and controller parts like the microprocessor chip are usually only available from the manufacturer of the oven. HV Capacitor: $10-20. they are not readily available from the common replacement parts sources.

and to minimize fire hazards.exact same current rating and at least equal voltage rating. For the repair. can I substitute a part that is not a precise match? Sometimes.org/sam/micfaq. radiation emission. This would be particularly desirable where your oven has one of those chassis mount $50 dollar varieties . However. There. Fortunately. 2. Magnetrons . Interlock switches . while an exact match may be required. However. it will heat up very
http://www. the value should be matched fairly closely or else other parts may be overloaded. a smaller one can be used for testing. You can safely substitute a not exact match for testing purposes IF you can make it fit the waveguide securely without gaps. 4. Many of these are interchangeable.it may be acceptable to use a $2. This will probably be a ceramic 1-1/4" x 1/4" 15 or 20 A 250 V fast blow type. You must be able to mount it securely and flush against the same surface as the old one. Creative mounting may be required.. For microwave ovens such parts include the power fuses. orientation of the cooling fins.a large number of microwave ovens use the same basic type but the mounting arrangement .75 generic replacement. this is simply desired to confirm a diagnosis and avoid the risk of ordering an expensive replacement and/or having to wait until it arrives.most parts are interchangeable. interlock switches. Fuses . and anything else that could potentially lead to microwave radiation leakage . Relays and triacs . except for a slight decrease in power output as the oven is used over the years and the magnetron ages. The power output is related to capacitance. Of course. For safety related items. However. Note that the working voltage rating of these capacitors is not consistent with the way capacitors in other electronic equipment are specified and is usually the RMS voltage of the AC input from the HV transformer.fore. Thus the organs from that carcass may be able to provide renewed vitality to your ailing microwave.substitutes will generally work as long as their specifications meet or exceed those of the original. 3. it doesn't have to be from the original manufacturer . differ. 6.Notes on the Troubleshooting and Repair of Microwave Ovens
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With the prices of microwave ovens dropping almost as fast as PCs.holes vs.similar (within 5%) and at least equal working voltage. a few year old oven may not be worth fixing if the problem is a bad magnetron or touchpad.same temperature and maximum current rating. use an exact replacement. it is not possible to substitute something from your junkbox unless it is from a microwave oven. Thermal protectors . HV diode .
Interchangeability of components
The question may arise: If I cannot obtain an exact replacement or if I have another microwave oven carcass gathering dust. you can expect a repaired oven to behave just about like new. if the cooling fins end up being on the wrong side. this is one situation where higher capacity (uF) is not better.most of these have similar electrical ratings so a substitution is possible if you can make it fit physically. there is little to go bad or deteriorate. HV capacitor . Here are some guidelines: 1. Therefore. a secure fit is very important as well for it to perform its safety function. the answer is generally NO . etc. a similar type may be used.like a magnetron which did not fit the waveguide properly. In addition. 5.repairfaq. 7.htm
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. Thus. For testing only.an exact replacement part is needed to maintain the specifications within acceptable limits with respect to line isolation. studs.must have the same terminal configuration and at least equal current rating.

50% of the input power goes to heat . Too small a capacitor and the doubler will not produce full output. however.speed/power and direction must match and mounting must be possible. 14.org/sam/micfaq. 11. As an example. the voltage rating must be at least equat to that of the original.
http://www. 9. Mounting should not be a problem but don't just leave it loose . It can be higher but never never lower or you will probably be replacing it again in the very near future. thermistors.Notes on the Troubleshooting and Repair of Microwave Ovens
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quickly .5 K ohms. These are really really rough calculations. I have not confirmed this. . Turntable and mode mixer components . Speed isn't so critical for a turntable but for a magnetron cooling fan. 8. Common shaded pole type motors may be interchangeable with other appliances or if a mounting arrangement can be cobbled together. Forget about the controller ICs or display. Observe the color code! 16. Cordsets . Controller and touchpad .small parts like resistors.I estimate that it is about a 1:4 change . 13.
Can I substitute a slightly different HV capacitor for a blown one?
It is not always possible or convenient to obtain an exact replacement high voltage capacitor. 15. Temperature sensors. Therefore. and so forth can often be substituted. capacitors. 10.same (within 5%) voltage and at least equal current rating.must be 3 wire heavy duty grounded type. The touchpad is likely to be custom both electrically and physically as well unless you have a similar model microwave to cannibalize. The cap is in effect in series with the magnetron. Light bulb . use them.you could end up with a disaster. A 1 kW magnetron running on just over 3 kV RMS is about 10 K ohms. the capacitor in a microwave is in a voltage doubler and effectively in series with the load (magnetron).5% (assuming the relationship is linear right around the nominal value).compatible switching and mounting arrangement. inadequate air flow will result in overheating and shutdown or failure. etc.and will not be suitable as a permanent replacement. Now for the uF rating: Unlike a conventional power supply filter capacitor.htm
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.if they fit. What will the effects be of using one that is a slightly different value? First. Mechanical timers . 12. its value **does** have an impact on output power.depends on the particular model. Fans and motors . Mica waveguide cover .cut to match.as well as heat dissipation in the magnetron. diodes.similar ratings and base. Make sure the replacement has at least as high a current rating as the original.increase the capacitor's uF rating by 10 percent and the power and magnetron heat dissipation will go up by 2. Thus the power difference is not a straight percent for percent change .repairfaq. HV transformer . A larger capacitor will slightly increase the output power . the impedance of a 1 uF capacitor at 60 Hz is about 2.

places like Digikey. However. and Newark do not have the specialized parts like magnetrons. may refuse to sell you anything microwave oven related due to safety concerns . needed for microwave oven repair. In the U. :)
Sources for replacement microwave oven parts
See the document: Major Service Parts Suppliers for some companies that I have used in the past and others that have been recommended. Then. The following suppliers have web sites with on-line catalogs and list a very extensive selection of microwave oven parts. I had to give up on a Sharp microwave/convection oven that was 15 years old because specialized replacement parts were no longer available from Sharp. it is definitely worth checking as the public web sites implie a desire to deal with the entire Internet community. interlock switches. thermal protectors. However. However.Notes on the Troubleshooting and Repair of Microwave Ovens
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Therefore. the cost will be higher than for generic parts from the places listed below if they carry what you need. I suppose this may be due to several factors including the potential liability issues. Sears doesn't manufacture their own appliances. Another alternative is to determine who actually made your oven. There is a chance that they may not want to sell to the general public. Many companies will be happy to sell service parts but availability may be a problem on older ovens. They may include microwave oven parts in their catalog but don't specialize in them.S. most electronics distributors will have a sufficient variety at reasonable cost. but an inspection inside may reveal the actual manufacturer.you have to convince them that you have at least the intelligence of an average carrot and possibly sign a 100+ page document written by too many lawyers. HV capacitors and diodes. Their prices are inflated as well. complaints/attempts to return parts when a repair doesn't work. nearly 50 different magnetrons are listed along with little photos of each!
http://www. Even Radio Shack can be considered in a pinch.repairfaq. whatever that might mean. etc. Allied. Note: I have heard that in other parts of the world. and the small quantities involved. there may be restrictions on who can actually purchase microwave oven parts other than things like light bulbs.com/ Their web site includes a very extensive selection of microwave oven parts. and standard door switches. This is obvious with name brands like Panasonic and Sharp. Global/MPI/All Appliance Parts Phone: 1-800-325-8488 Web: http://www. go direct to the horse's mouth. turntables. Also.unless they are convinced you are a certified repair technician. However.. For example. Your local appliance distributor or repair parts outlet may be able to obtain an exact replacement or something that is an ecceptable substitute.
Obtaining replacement parts for microwave ovens
For general electronic components like resistors and capacitors. certain companies (like Sears) may set their own rules .org/sam/micfaq.allapplianceparts. I would say that using a capacitor with up to a 10-15% difference (either way) in uF rating is probably acceptable but a closer match is better. some places like Sears.htm
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. Going direct to the manufacturer is a possibility but expect to pay more than might be charged for generic replacement parts by an independent company..